Free Ghoul prisoner

Free Ghoul prisoner is an unmarked quest in Fallout.

Detailed walkthrough
In the buildings of the Watershed area of Necropolis, the Vault Dweller will find some prison cells just down from the water pump. One of the cells contains a ghoul prisoner.

The Vault Dweller can unlock the cell and ask the inhabitant about where the water chip would be kept. Conveniently, the entrance to the vault where it resides is right next to the ghoul's cell. Experience is gained as soon as he is spoken to.

Rewards

 * 500 XP
 * Location of water chip - Vault 12

Bugs
If you talk to the ghoul prisoner before killing the super mutants and then leave the area without freeing him, he will only say "Thank you again." if you return even if you have not unlocked the cell yet, making the side quest impossible to complete.