• This is a guide for the mod Black Mesa: Blue Shift which you can find in the workshop. You should definitely play it normally before trying this challenge (it goes without saying that this guide has spoilers for most of the mod, so read at your own risk).
  
  
• I´m sorry beforehand for the quality of the screenshots, my computer isn’t the best to play, though I did my best to take them at the best possible quality.
  
  
• This is the first guide I make in English, which isn’t my mother tongue. So, I hope I can make myself clear as you read it (if you see any typo, leave it on the comments so I can correct it, thanks!).

Just like in the original Half-Life: Blue Shift, if you break the box in Barney's closet in Chapter 2, you'll find a Chumtoad. This one, however, is a plush toy that you can grab and carry around freely. Our goal is to get it to Xen with its siblings.
The plush has some properties to consider in our mission:

• It floats underwater all the way to the surface.
  
• It doesn´t roll like the hat in Black Mesa.
  
• Its size makes it tricky to get through ladders (although, fortunately, there aren’t that many of them unlike Black Mesa).

And now, without further ado, let’s begin.

Start a new run in Chapter 2 and go to Barney´s locker. You´ll see a box with a skull and crossbones symbol which we´ll have to break. To do that, carry it to the shooting gallery, pick up the pistol and shoot it open.
From here, take the plush to the end of the chapter and leave it in the elevator with Patrice and Trevor.
Fix the elevator and finish the chapter normally.

When you wake up in the elevator, you´ll see the plush on the right seat next to a dented helmet.
Pick up the plush and leave it next to the door with electrified water. Now, you´ll have to deactivate the electricity and return here to keep going.
Later, you´ll find a scientist trapped behind a door. Leave the plush there and explore the facilities to find a security officer. Afterward, you'll return to the scientist and be able to continue forward.
After leaving Murdoch behind, you'll come to a ladder. To climb it with the plush, you have to get close enough for Barney to grab onto it, turn around, and climb facing backward and upward.
Remember to do this everytime you find a ladder.
After crossing a ventilation duct, you'll reach a canal where you'll have to cross to the other side using a broken container.
There are two options: leave the plush inside the container and return to it when you activate it to cross, or you can climb a handrail, run, and throw the plush to the other side. Choose the one you find the most comfortable.
After reaching the tunnel with the crashed car, leave the plush outside the security office to open the gate next to it.
After opening the gate, go to the roof of the crane control room and leave the plush next to the red toolbox along the way. Once you finish the puzzle, pick it back up and cross to the other side of the tunnel.
After crossing, you can throw the plush below the fire door or you can carry it with you through the vent (both take you to the same place).
When you arrive to the waste disposal, you can just leave the plush in the control room and go back to it after blowing up the grinder.
Right at the end of the chapter, you´ll get to a ladder that takes you to the surface. Repeat the same technique I mentioned previously to climb up and finish the chapter normally.

Advance normally until you reach the glass tunnel where you find the Magnum. Leave the plush there and find a way to blow the access to the steam tunnels.
When you reach the hallway with steam blocking the path, you can leave the plush there and solve the leak.
Later, you´ll reach the drainage basin. Leave the plush next to the gate blocked with steam and blow the pipes to clean the path.
The next area is a little tricky because it's a flooded tunnel with fans trying to suck you in. To get the plush through, you have to swim to the right as soon as you dive into the water, climb onto the crates, and throw the plush over the fence.
After doing this, you can carry on without worrying about losing the plush.
From now on, you´ll have to fight your way to Rosenberg while carrying the plush with you.
Don´t worry about the plush in the sequences where you fall from a container and the one where the HECU throws you inside Rosenberg´s container. As long as the plush is near you, everything will be fine.
Once you reach the prototype labs with your team, wait until Rosenberg and Simmons open the portal and jump in while holding the plush (you´ll see it travelling with you).

Upon arriving at Xen, the plush will appear next to you after being attacked by a Knockback Houndeye.
Keep going normally through Xen and, when you arrive at the image´s leaves puzzle, leave the plush nearby and solve it.
Later, you´ll reach an open area with Alien Trampolines and another leaves puzzle. Leave the plush in the zone with an Alien Trampoline surrounded by scientific tech and solve the puzzle.
After that, you´ll get to a pond with "Archers", a cut enemy from Half-Life. The most efficient way to kill them is waiting for them to start charging their long range electrical attack (when that happens, they´ll start shining like the second image) and shooting them with the pistol.
Later, you´ll reach the exterior of Site Alpha - Primary Habitat. Leave the plush near the first Alien Trampoline you find (the one that´s pointing to the building) and clean the column to then go back and enter the base.
Once inside Site Alpha, advance normally until you get to the building´s Main Power Supply. Just leave the plush there and pick it up once you charged all batteries to the generator.
We´re almost there!
After arriving at Site Beta - Resource Extraction, you´ll fall from a waterfall to a big pond with Archers and Barrel Cacti. Leave the plush near the Healing Pool, blow the Cacti underwater and kill the Archers (use the method stated before) to advance.
After blowing the third group of Cacti, you´ll see a cave blocked by rocks. The Chumtoad´s Lair is behind them.
Shoot at the Cacti behind the rocks and bring back the plush with you to enter the cave.
And ladies and gentleman, we finally arrived to Chumstoad´s Lair!
When you enter the lair with the plush, a device will rise from the waters. Put the plush in it and claim your prize for all your hard work.

After reuniting with Murdoch and Otis in Captive Freight, Murdoch comments about us carrying the plush everywhere.

If my guide was useful, please, consider giving a like and adding it to your favorites. Thank you so much! :)