So the story sets off during MvM times, meaning somewhere around 1970s if you wanted to be specific about it. The battle against these bots is still raging on, with another carrier tank being emptied out of robots, forcing it to retreat.

However, when it does so, another carrier takes its place, though this time a lot larger and a lot... greyer. The mercs think nothing of it at first, possibly just a few more robots and a forgotten coat of paint. However, things quickly take a turn when the robots that come out are stronger and smarter than others, being able to start taking over mann co.'s many bases.

Due to this big problem the engineer and medic quickly rush off to try and work out a way to somehow stop them before it's too late, though due to the loss of two men on the frontlines many of the other mercs end up captured, only having the sniper left uncaptured due to his post being much further back.

The goal is simple: you must go out and collect the required machine parts the engineer and medic need for their experiment, rescuing your friends along the way (and unlocking the ability to play as them as well) before it's too late.

The first map, as you would expect, is coaltown. The map is expanded to have the mining town larger, have some buildings enterable and have the mine actually open for you to explore. You also save the soldier here quite early on as well as the heavy a bit later.

Decoy is the second map, having more towering buildings and cliffs to explore. Demoman can be unlocked here as well, which is cool.

Bigrock is the third map that you can access, with the caves and open areas expanded a LOT. Luckily though scout is unlocked here so you can get around easier if you want to go out exploring.

The next map you visit on your quest is Rottenburg, where you will find the medic outside helping you look for parts. He gets captured and, after you save him, you can play as him as well. Pyro can also be found here. The expanded areas are mainly the field at the start being bigger, the cliffs being climbable and the town being increased greatly in size, having some parkour challenges and hidden secrets.

Mannhattan is the fifth map you have to go to on your quest for parts. This is where you also unlock the last two classes, Engineer and Spy. Places like the train station, dock, gates and even the tunnels below the spawn-area bridge can be explored in great detail.

The sixth, and one of the most difficult areas, is Mannworks. The map itself is simply enhanced greatly in size, with more buildings that sometimes involve lots of climbing into nauseatingly high areas, climbable cliffs that lead to complex forest mazes and a lot of obstacles around to test your skills out. No new characters are unlocked since you have them all at this point. This is also where the game's story comes to a close in easy and normal difficulty.

Ghost Town, however, is only accessible by clearing all the other areas 100% and being in hard or expert difficulty. It is a reskin of this game's coaltown, much like how ghost town was in normal TF2, but this time with a LOT more difficulty thrown at you. This is also the special final area if you were playing on those harder difficulties and, if you weren't, you can't get here.

The game itself would mostly be a mixture of challenge and fun, not being frustratingly hard nor being a cakewalk. Of course the difficulty increases as you go from area to area, though only has one or two spikes in the final zones to test your skills.

The difficulty settings are chosen at the start of each game and cannot be changed once set (though you get ~3 saves so you'll most likely get to try them all out anyways, plus if you can't just erase an unused save file if you want to try something easier or harder.)

Easy Mode: Classes have more health, less collectibles, less complex challenges, foes given less health and slower attacks and bosses have less health and easier patterns.
Normal Mode: Everything as it was intended, with all stats normal
Hard Mode: They mainly just do small things like reduce time in time trials, make foes a bit smarter and give them more health and for bosses they just add more health and a new attack or two to throw you off.
Expert Mode: So expert mode is a ragefest. Most challenges require you to be almost perfect to even stand a chance, foes are buffed up significantly with more health and often better weapons, classes have less health, bosses are given complex patterns and whole new phases to their battles and random crits exist here. (Okay the random crits are not always random also yes they do kill all classes but Heavy in 1 hit)

The difficulties also link to achievements which will probably be talked about more at the end.

Sniper:
Health: 5, Speed: Average, Weapon: Kukri
Special Ability: None
Special Challenges: Sniping Alley
"Nothing too special since he's the starter character. Though he's balanced overall and good for learning the ropes without gimmicks coming up."

Soldier:
Health: 6, Speed: Slow, Weapon: Shovel, Mantreads*
Special Ability: Damage foes you land on, dependent on falling distance
Special Challenges: Rocket Launcher Rumble
"Sacrifice speed for a bit of health and a mantreads effect. Nothing too extreme but can be useful on some areas if there are a lot of enemies below you."

Heavy:
Health: 8, Speed: Very Slow, Weapon: Fists, Sandvich*
Special Ability: Have a sandvich to eat every 30 seconds to restore 2 health, immobile while eating
Special Challenges: Boxing Match
"Slow but sturdy and complete with a portable health pack. The heavy is a pretty solid pick for areas crawling with enemies, though he isn't so good when it comes to parkour."

Demoman:
Health: 6, Speed: Slow, Weapon: Bottle, Chargin' Targe*
Special Ability: Charge forward to damage foes every 10 seconds, cannot turn and charge lasts 1.75 seconds or until collision.
Special Challenges: Rapid Reactions
"Like soldier, but with a charge instead of mantreads and no depth perception instead of no sight at all."

Scout:
Health: 3, Speed: Very Fast, Weapon: Bat
Special Ability: Double jumps that can be performed at any time with no cooldown
Special Challenges: Races
"Sacrifice health and damage for the best mobility in the game"

Medic:
Health: 4, Speed: Fast, Weapon: Bonesaw
Special Ability: Slowly regenerate back life, getting back roughly 1 health every 30 seconds
Special Challenges: Uber Challenge
"A bit less damage and a bit less health though in return you can regenerate health without the need of health packs"

Pyro:
Health: 5, Speed: Average, Weapon: Fire Axe, Sharpened Volcano Fragment*
Special Ability: S.V.F. Swing with extra damage and afterburn every 20 seconds
Special Challenges: Flamethrower Fury
"Extra damage but a slower swing speed, also comes complete with the only clip damage in the game with Pyro's ability"

Engineer:
Health: 4, Speed: Average, Weapon: Wrench, Gunslinger*
Special Ability: Place down a low-health mini-sentry every 40 seconds with infinite ammo
Special Challenges: Tower Defence
"Give up a bit of health but in return get the ability to throw down a mini-sentry for ranged damage occasionally"

Spy:
Health: 3, Speed: Fast, Weapon: Knife
Special Ability: Cloak meter and a disguise kit to fool enemies. Backstab normal foes for insta-kills.
Special Challenges: Stealth Sections
"Good for getting around without battling too often. Not so good in large fights"

Special Abilities: Things that your character can activate at any time as long as they have recharged
Special Challenges: Minigames that can either be played for fun or for special collectables at certain areas on maps

Sniper:
Special Ability: None
Special Challenges: Sniping Alley - Like Duck Hunt, but with better graphics and shooting robots instead of ducks

Soldier:
Special Ability: Damage foes you land on, dependent on falling distance
Special Challenges: Rocket Launcher Rumble - Run around a small arena using a rocket launcher to wreak havoc with a top-down perspective

Heavy:
Special Ability: Have a sandvich to eat every 30 seconds to restore 2 health, immobile while eating
Special Challenges: Boxing Match - Punch Out!, but with Heavy punching big robots instead

Demoman:
Special Ability: Charge forward to damage foes every 10 seconds, cannot turn and charge lasts 1.75 seconds or until collision.
Special Challenges: Rapid Reactions - Wait for the '!' and press the attack button as quickly as you can, if you press it before the foe can react you win. Best out of 3.

Scout:
Special Ability: Double jumps that can be performed at any time with no cooldown
Special Challenges: Races - Race a Scout robot across the map to a goal, with both sides able to hit the other from behind to slow them down

Medic:
Special Ability: Slowly regenerate back life, getting back roughly 1 health every 30 seconds
Special Challenges: Uber Challenge - You charge up an uber by ~5% per second and then, with full uber, you get to 'go to town' on your enemies with damage immunity for 10 seconds.

Pyro:
Special Ability: S.V.F. Swing with extra damage and afterburn every 20 seconds
Special Challenges: Flamethrower Fury - Like Soldier's special challenge but with Pyro and a flamethrower

Engineer:
Special Ability: Place down a low-health mini-sentry every 40 seconds with infinite ammo
Special Challenges: Tower Defence - Like normal MvM but with sentries instead of real teammates

Spy:
Special Ability: Cloak meter and a disguise kit to fool enemies. Backstab normal foes for insta-kills.
Special Challenges: Stealth Sections - Use the cloak and disguise to sneak past the robots and reach certain places and items, sometimes it is timed so you can't just stall the cloak's recharge behind a wall

Scout
Health: 5, Speed: Fast (Giant HP: 40)
Type 1: The scouts only use their bats, though their swing speed is slow
Type 2: Same as the first, though they swing faster

Soldier
Health: 20, Speed: Slow (Giant HP: 65)
Type 1: Aim at you with the rocket launcher, then fire. Cannot turn once locked into a direction
Type 2: Like the last kind but can continue turning
Type 3: Fires bursts of slow rockets instead of just 1 at a time
Type 4: Type 1 but with direct hits instead

Pyro
Health: 10, Speed: Slow (Giant HP: 50)
Type 1: Use fire axes, not too dangerous
Type 2: Use flare guns, like the type 1 soldiers but slower firing and faster projectiles
Type 3: Use flamethrowers but there's a small delay between aiming and firing
Type 4: Spam the flamethrower, just keep rotating 360 degrees constantly

Demoman
Health: 15, Speed: Slow (Giant HP: 60)
Type 1: Fires grenades, similar to type 2 soldiers
Type 2: Makes sticky traps for you to navigate, cannot attack unless you touch a stickybomb
Type 3: Demoknights that charge at you occasionally and swing horizontally for more melee coverage

Heavy
Health: 30, Speed: Very Slow (Giant HP: 80)
Type 1: Slow-firing punches, similar to slow, bulky scouts
Type 2: Aim with the minigun and fire if they see you for 3 seconds or more

Engineer
Health: 10, Speed: Average (Giant HP: 100)
-Sentries work like type 2 heavies, though with 20 HP
-Teleporters and dispensers both have 15 HP
Type 1: Teleports robots in and tries to flee from the player, if it dies the teleporter will too
Type 2: Builds only dispensers, same rules as type 1
Type 3: Builds sentries only, same rules as type 1
Type 4: Builds everything, though it is destroyed once he dies

Medic
Health: 10, Speed: Fast (Giant HP: 45)
-Regens 1 health every 2.5 seconds
Type 1: Heals an enemy with the medi-gun for 1 health per second
Type 2: Heals 2 health per second
Type 3: Heals 10 per second, needs to be killed to damage whoever it is healing

Sniper
Health: 5, Speed: Average (Giant HP: 30)
Type 1: Blue sightline, fires slow-moving bullets
Type 2: Huntsman, no sightline and a larger projectile

Spy
Health: 5, Speed: Average
Type 1: Invisible most of the time, tries to backstab the player for 3 damage

-Most bosses have the camera zoomed out, usually giving the player the view of the whole arena in smaller arenas or at least most of it in the case of some of the larger arenas, giving the layer plenty of time to react. However for some bosses, like the Mannworks 2nd Mini-Boss this doesn't happen

Brigadier Bazooka
Fought inside Coaltown's mine as the first boss. He is fairly easy since he is the first boss.
Weapons & Cosmetics: Beggar's Bazooka and Soldier's Stash
Health: 30 (Easy), 50 (Normal), 65 (Hard), 80 (Expert)
-Phase 1: Sits in the background, summoning forth soldier robots to attack for him. This phase doesn't happen in easy mode, he summons 1 and then 2 in normal (3 total), 1, 2 and 2 in hard mode (5 total) and 1, 2, 3 and 4 in expert (10 total). In hard and expert he also fires onto the arena from the sides, 1 rocket at a time in hard and 3 in expert
-Phase 2: Jumps into the arena and proceeds to fire at the player. Fires a rocket every 3 seconds in easy, 2 every 2.5 seconds in normal, 3 every 2.5 in hard and 3 every 2 seconds in expert. The rockets go horizontally, not exploding onto the arena but crossing it for a higher chance to hit you.
-Phase 3: At 10/20/30/35 health he jumps to the back wall of the arena and fires off burst shots with the bazooka. Easy fires 2 every 4 seconds in easy, 3 every 3 in normal, 3 every 2.5 in hard and 4 every 2 seconds in expert.

Grand Demorai
Decoy's boss is a large demorai. He focuses mainly on melee attacks, making him tricky to attack
Weapons & Cosmetics: Samur-eye, Grenade Launcher, Splendid Screen and Half-Zatoichi
Health: 40 (Easy), 60 (Normal), 85 (Hard), 115 (Expert)
-Phase 1: Fires 3 (Easy), 4 (Normal/Hard) or 5 (Expert) grenades onto th arena from the corner and begins laughing. Laugh depends on difficulty.
-Phase 2: At 30/40/55/70 Health it throws away the grenade launcher and gets out the zatoichi. He charges across the arena, getting stuck in the walls to give you opportunities for damage. Breaks between charges dependent on difficulty.
-Phase 3: At 15/25/30/40 Health he runs around, swinging the zatoichi. The pattern is 2 horizontal and then a downward in easy and normal and 3 horizontal in hard or expert. Downward swings get the sword stuck, giving you time to deal damage.

Gold Gauntlet
Bigrock's boss is a large engineer bot with golden weapons and a bad singing habit. He has the gold chains from the triad trinket around his neck and a patriot peak on his head.
Weapons & Cosmetics: Triad trinket, Patriot Peak, Rescue Ranger(?) and an Australium Wrench
Health: 50 (Easy), 65 (Normal), 100 (Hard), 140 (Expert)
-Phase 1: Fires rescue ranger bolts from the middle of the arena. They get stuck in the ground and then violently explode shortly after. Some shots are directed at the player and some are just spammed elsewhere, making an explosive ring.
-Phase 2: At 20/25/40/50 health the engineer's deadly daffodil hypno beam eyes start up, he sings again and a storm brews, making the screen much darker. The phase consists of him teleporting using special instant-teleporters and slamming his wrench down into the ground after teleporting, laughing for a short time after every swing. The laugher is when he is vulnerable, though it is only for a second or two.

Py-Robo
Rottenburg's boss is a large pyro with a large flamethrower that appears in the courtyard
Weapons and Cosmetics: Rusty Reaper, Degreaser
Health: 70 (Easy), 90 (Medium), 120 (Hard), 165 (Expert)
Phase 1: Jumps onto the cliff and sends a few random robots at the player. Upon their defeat it jumps down and walks forwards, spraying the flamethrower. The direction will only ever lead to another cliff. Once it reaches the cliff it'll jump back up and summon more robots and repeat the cycle.
Phase 2: At 45/60/95/130 Health it jumps into the middle of the arena and electrifies itself, locking onto the player's locations and spraying flames. In the corners of the arena sometimes rocks will fall from the cliffs which can be hit, causing them to roll forwards. If they hit the Py-Robo it stun it and makes it drop the electric barrier, giving you a while to hit it before the shield goes back up and the cycle repeats.
Phase 3: At 15/25/40/65 Health it begins to jump into the air, land where the player was standing and then either fire 3 large fireballs that keep rolling until they hit the walls or it sprays the flamethrower forwards. After doing either of those 2 things it has a few seconds before it jumps back up again and with that the cycle repeats itself until the boss is beaten

The Tank
Mannhattan's mini-boss is a tank which will burst out of the wall at the start and drive towards the player, though turns slowly.
Health: 50 (Easy), 65 (Medium), 80 (Hard), 100 (Expert)
Phase 1: In this phase it just drives at you, trying to flatten you. You have to use its slow turning speed against it by getting to its sides and hitting them to avoid being flattened.
Phase 2: At 20/30/40/50 health it quickly rushes into one of the four loading doors at the top, bottom and sides of the arena. It will charge in-between all of these for the rest of the fight, either crossing the arena vertically or horizontally. It hangs just outside the gate before each charge, giving you a bit of time to hit it if you're quick.

Soldered Soldier
Mannhattan has a soldier-exclusive boss which is a soldier riding inside of the tank fought previously. It attacks mainly using the tank which, since its last encounter, has recieved both repairs and upgrades.
Weapons and Cosmetics: Armoured Authority and Rocket Launcher (Soldier), Rocket Launcher and Grenade Launcher (Tank)
Health: 75 (Easy), 100 (Medium), 135 (Hard), 180 (Expert)
Phase 1: Same as the tank's phase 1, with it driving at you
Phase 2: At 50/80/100/135 health the tank's top opens, revealing the soldier inside. The soldier will now occasionally fire a rocket launcher at the player or get the tank to charge. When the tank changes it crashes, stunning it and the soldier inside, which is when you get a chance to hit it for ~4 seconds
Phase 3: At 30/50/65/90 health the soldier gets back inside and the tank's sides and front open up to reveal twin rocket launchers at the sides and a grenade launcher at the front. The rocket launchers fire forwards and the grenade launcher fires at the player, acting identically to the normal TF2 one with the rolling pills. It once again has to charge in order to become vulnerable, though once charged all of the weapons temporarily shut down.
Phase 4: At 15/25/30/45 health the soldier comes back out and it becomes a mix of phases 2 and 3, with all 3 weapons and the soldier firing at you. Has to still charge and hit a wall to become vulnerable, though this time it will reactivate in 2 seconds rather than 4.

Elite Bowman
Mannworks' sniper-exclusive mini-boss is a giant huntsman-wielding sniper boss. You're both on moving platforms in a 3x3 arena, with the robot using rope-wrapped arrows to grapple around and you using buttons on the sides of your platform.
Weapons and Cosmetics: Trophy belt, Huntsman
Health: 9 (Easy), 12 (Normal), 15 (Hard), 21 (Expert) (Only 1 damage per hit)
Phase 1: Grapples onto a wall and pulls itself towards it, destroying your platform if it hits you (instant-death btw) before firing 1/3/4/5 arrows in an arc at the player. Jarate will then appear in the middle of the arena which you can pick up and throw at him as he taunts, causing him to spark wildly before temporarily deactivating, giving you a chance to it 3 times before it starts up and briefly electrifies its platform (and yours if you don't get away again).
Phase 2: At 3/3/6/12 health the pattern is mostly the same but a bit faster and instead of the sniper firing the arrows holes in the walls appear and snipers appear from them and fire at the player, being either 3/4/5/6 arrows. The wall the hole appears from changes every cycle.

The Challenge Champion
Mannworks' main mini-boss is actually a collection of boss fights that use the class-specific special challenges as their mechanics, varying between each battle. The boss itself is actually just a modified heavy robot. You get checkpoints after every win btw.
Weapons and Cosmetics: Varying weapons, though always wears a team captain.
Sniper's part involves you having a sniper duel with the robot. You have 5 seconds to locate and shoot it before it shoots you. It has 3/4/5/8 health whereas you only have 1.
Soldier's part is basically a bullet-hell against the heavy robot, with you both firing rockets (though the robot gets to make complex patterns while you can only fire forwards). You have 3 health while the heavy has 20/30/45/60
Scout's part is a race against it with similar vibes to Metal Sonic's Sonic CD boss battle. The heavy has a rocket booster on its back so it can go fast and a soldier robot with a laser chases you both. The loser gets stuck outside the door and thus gets zapped by the laser. The heavy cannot be slowed by attacks.
Demoman's part is the reaction game, though you have to win 3 times and the robot has quite fast reactions.
Spy's game involves sneaking behind it and backstabbing it 3 times, destroying the rocket in the process. If it spots you it will rush up to you and insta-kill you so stay out of its line of sight.
Pyro's part has you spraying it with the flamethrower as it lumbers after you, trying to punch you constantly. You need to hit it quite a lot with the flamethrower, though it only needs to hit you once.
Engineer's part involves you setting up sentries that slowly drain its health by shooting it. It will attack whatever is closest, whether it is a sentry or the engineer, destroying whatever it hits in 1 punch. Shooting it enough will defeat it.
Medic's part has you charging up your uber (~5% per second) while staying away from it. One uber has been achieved you have to go up to it and just spam the boss constantly with attacks. It takes 10/20/40/70 hits to defeat it, with you getting in ~10 hits per uber.
Heavy's part is the final one where you have to face it in a boxing match. It can punch downwards to the sides or it can try and headbutt you where, if you dodge, it stuns it and gives you a chance to punch it in the face a bunch of times. You need 30/45/60/90 punches to defeat it (6 per stun, can't hit it any other time) though it only needs to hit you 3 times. Once you beat it here the boss battle ends.

Heavy King
Mannworks' main boss is a colossal heavy robot wearing a crown. It is so large that most of the buildup to the battle involves you circling it on scaffolding to reach the top.
Weapons and Cosmetics: Class Crown, Fists, Rocket Launchers, Flamethrower, Sawmill's Sawblade.
Health: 100 (Easy), 145 (Normal), 180 (Hard), 240 (Expert)
Phase 1: In phase 1 the boss is pretty simple: It slams one of its large hands into the ground, trying to crush you. You then get an opportunity to hit it about 2 or 3 times before it raises its hand again for a 2nd attempt.
Phase 2: At 65/90/120/175 health it changes up its pattern, adding in a few new attacks. Its cycle changes to it firing several rockets into the air that land in a spiral, forcing the player to run away as the rockets try to land on them. It then fires a rocket that circles the whole arena you have to jump over a few times before it flies off into the aether. Finally it slams its hand down again which stays there for longer to compensate for the increased cycle.
Phase 3: At 30/50/65/100 health it adds in a few more attacks and also randomises them all. It can now put down both its hands at the sides to trap the player (they become electrified so you can't hit them immediately) before spraying flames from its mouth the player must either jump over or duck under. After a while the hands' shields go down so you can get in a few hits. It also fires a few extra rockets when making them rain down from above. The other attack it gets is that it can summon a sawblade from its chest that travels across the platform that waves from the platform into the foreground. To dodge it you have to walk past it while it is in the foreground since it's too big to jump over for anyone but the scout.
Phase 4: Finally at 10/20/30/45 health it becomes enraged and smashes down most of the scaffolding except a single screen-wide chunk where you are. The flamethrower, sawblade, punch and horizontal rocket attacks don't really change but the 'rocket rain' only covers ~75% of the platform and falls a bit faster to compensate. Defeating this phase will defeat the boss and cause the large heavy to explode and collapse

The Boss Rush
Ghost Town's bosses are, for the most part, stronger versions of previous bosses, making their attacks faster and doubling all of their HP bars, though the Soldered Soldier's boss is nowhere to be seen.

The Soldered Soldier
The true final boss who appears as Ghost Town's main boss. It looks the same, though a bit damaged from your last encounter.
Weapons and Cosmetics: Armoured Authority and... most weapons.
Health: 350 (Hard), 700 (Expert)
Phase 1: In phase 1 it grabs a large Black Box and puts it on its back before flying off of the platform. It attacks by firing barrages of rockets down onto the platform, occasionally landing in-between to reload and recharge, where you can freely get some hits in.
Phase 2: At 300/600 health the rocket pack explodes so he pulls out a flamethrower. Mostly he just walks towards you slowly, spraying the flamethrower constantly. Occasionally the flamethrower will jam and he will stop and try and fix it, where you can get a few hits in before he does.
Phase 3: At 250/500 health he pulls out a stickybomb launcher and then proceeds to fire stickybombs over most of the arena while invulnerable. He will accidentally hit hit shield with one each time which is important in a second. He then detonated them all in the order they were spawned, with the last one being the one stuck to his shield which deactivates it, giving you time to hit him again.
Phase 4: At 200/400 health he jumps down off of the platform and into a giant robot which, while not quite as large as the Mannworks boss, is still larger than most giants. It will fire grenades from its chest that roll along the platform and then all explode after a while, then jump up onto the platform and try to flatten you by stomping on you. Eventually it stops to taunt where you can hit its feet a few times before the cycle repeats.
Phase 5: At 150/300 health its feet and legs explode so it uses a rocket on its back to stay mobile, quickly vacating the main platform. Now it can use the rocket launcher attack from phase 1, the grenades from phase 4 and a minigun which you can only avoid by constantly moving. Sometimes it fires its fist at the player which can be jumped on. When the fist retracts you can hit it a few times in the head before being flung back onto the platform.
Phase 6: At 100/200 health it stops doing the rocket punch attack but gains two new attacks. It can now spawn 4 sawblades from its chest that slightly home in on the player as they roll and it can also fire lasers from its eyes that quickly travel across the ground before the laser's trail flares up and ignites briefly (The laser and the trail don't hurt but the fire does). It will now sometimes land on the platform to let the rocket cool down where you can hit it.
Phase 7: Finally, at 50/100 health its body explodes, forcing it to only use the laser attack. However, it now spams the laser attack constantly to compensate. Eventually it will run out of energy and fall into the platform breifly, where you can proceed to get in a few attacks. After running out of health the head explodes and the boss is defeated.

Mainly this just consists of a group of collectables that can be found in certain areas of the game. There are three different categories of item that can be found:

Rust:
Items that are relatively easy to get, can often be seen and collected just by walking around a little bit. Most areas have 20 of these items.

Silver:
Items that are only slightly harder to get, sometimes requiring looking in unlikely places or performing a bit of tricky platforming. Each area has 10 of these to get.

Australium:
The rarest of the secret items, being very well hidden and often in very hard to reach and seemingly pointless places most of the time. Each area has 5 of them to get.

Collecting these items, while technically pointless, will still net you a few potentially-cool things: For example, achievements for finding them and, with certain achievements, potentially some genuine hats as well.

A Journey Begins
Complete the tutorial

A Helping Hand
Free one of the other mercenaries

The Band's Back Together
Free all of the trapped mercenaries

Journey's End
Complete the game on any difficulty

Fancied A Challenge
Completed the game in hard mode

An Expert Arises
Completed the game in expert mode, the hardest difficulty

The Bigger They Are...
Defeat a boss

...The Harder They Fall
Defeat all of the game's bosses

Perfectionist
Complete all of the game's bosses without taking damage

Coaltown Commander
Finish everything there is to do in Coaltown

An Effective Decoy
Finish everything there is to do in Decoy

Best Of Bigrock
Finish everything there is to do in Bigrock

Rottenburg Rampage
Finish everything there is to do in Rottenburg

Mannhattan Projects
Finish everything there is to do in Mannhattan

Worked Through
Finish everything there is to do in Mannworks

The End?
Beat the Mannworks boss in either Hard or Expert Mode

Many Hours Wasted
100% the whole game...!?

Rusty/Silver/Gold Coaltown Collector
Find all the rusted/silver/gold collectables in Coaltown

Rusty/Silver/Gold Decoy Duster
Find all the rusted/silver/gold collectables in Decoy

Rusty/Silver/Gold Bigrock Bounty
Find all the rusted/silver/gold collectables in Bigrock

After certain bosses, events and achievements you can get special items for TF2... though most of these are just genuine hats...