I know a lot of people here don't want to support the Nacon version for various reasons. I'm not here to weigh in on the legal situation HOWEVER there are objective issues with the Steam release. The major one being that the Steam version is missing a few DLCs. After you follow these steps you will not have to worry about that anymore and you will be able to feel like you have supported the original dev.

NOTE: Please try to start from a fresh save and fresh installation in order for everything to work smoothly. This guide assumes that you are starting from scratch with fresh installs and no in-game progress.

• STEP 1: Purchase the Steam Version AND the GamesPlanet version (I know this part sucks but I have been able to snag both for under $25 by taking advantage of sales)
  
• STEP 2: Fully install both versions to separate folders (GP version requires extra steps)
  
• STEP 3: Navigate to the folder where you installed the GamesPlanet version and highlight everything and copy it to the folder where the Steam version is installed. If prompted choose to Overwrite and Replace everything.
  
• STEP 4: Now you have the GP version with all the DLCs launching in place of the Steam version but achievements will not be functional. We are going to fix this with a simple mod. So visit this page Nexus Mods - The Sinking City Achievements Unlock[www.nexusmods.com] and download the mod.
  
• STEP 5: Extract the 2 files from the archive to "The Sinking City\TSCGame\Content\Paks"
  
• STEP 6: You are done! Launch game and test with Fortune Teller or Finish first quest.

• Once you visit the hotel save your game and quit to the desktop.
  
• Navigate back to "The Sinking City\TSCGame\Content\Paks" folder and remove the 2 files we added with the mod.
  
• Reload your saved game.
  
• A letter related to the Necronomicon DLC will appear on the desk in your room.
  
• Pick up the letter, save and exit the game.
  
• Add the mod files back.
  
• You are done.

Some users are reporting having issues when following the methods listed in this guide after already making significant progress through the game.

ISSUE: After following this guide the DLC Quests are disabled. This is caused by the Achievements-unlocking mod linked in the guide above. It seems *so far* to only happen if you follow this guide after having made a lot of progress through the game.

SOLUTION:

• Step 1: Remove the Achievement mod files from the Paks folder
  
• Step 2: Start game and unlock the quests.
  
• Step 3: Add the mod files back to the Paks folder and Enjoy!

----Worshipers Of The Necronomicon DLC Quests----

• Mystic Tomes: Unlocked via your hotel room, you'll find the note "A Plea for Help" sitting on the desk in your room
  
• Breaking News: If you purchased the DLC, this side mission will become available at the Oakmont Chronicle, after you have completed the Nosedive Story Mission, and the Through the Looking Glass Side Mission. After you conclude the Nosedive mission, you must return and speak to Robert Throgmorton and receive your reward for that mission before this mission will become available.
  
• Blood From A Stone: It is available only after completing the Breaking News DLC mission. Speak to Helen Bland at the Oakmont Chronicle and ask... Got anything new for me to cover? then say... I'm in.

----Merciful Madness DLC Quests----

• Darling Of Fortune: Available at the police station after speaking to a guy named Jacob Tucker. You must have completed 6 main quests and Throgmorton's two quests.
  
• Defunct Brain Cylinders: Unlocked by looking at an urn in the Asylum basement with the Mind's Eye.
  
• Fool's Gold: Unlocked by picking up a piece of paper on the front desk of the Oakmont Chronicler (in front of Helen).
  
• Silence is Golden: (This quest may be part of the Steam release by default but I am still listing it here for posterity anyways because its listed as part of the Merciful Madness DLC on a few other platforms.) This quest is unlocked at the Oakmont University Library after you complete Nosedive.[/list]
  
  *Sorry if I've missed anything. Its been a while since I played and a lot of the guides online are incomplete and/or have incorrect information. If you notice any mistakes please feel free to let me know in the comments.*
  
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  Credit for bringing this issue and solution to my attention belongs to ♥*¨*★~Preda~★*¨*♥