After talking to the underground ghouls of Necropolis, travel to the watershed area and then enter the Necropolis sewers that are being guarded by Larry. In the eastern end of this section of the sewers lies a pile of junk. It is essential to the completion of the quest. Alternatively, junk can be more easily acquired from the Hub, found in a crate in the south-east corner of the weapons supply shop of Jacob in the Hub's Old Town.
The Vault Dweller then needs to approach the water pump and use the junk to fix the pump. A successful Repair skill check will fix the pump. A critical failure will remove the junk from the Vault Dweller's inventory and deal 1 point of damage to them.
An alternative way to get this quest is from Set, if the Master is still alive. If the Vault Dweller has already searched Vault 15 and is in possession of the "junk," Set will have his surly minion, Garret, fast travel to the water pump. The Vault Dweller is then forced to repair it at gunpoint.
The game only checks having any junk in the inventory, allowing one to use any such item (such as from Jake's house in the Hub) to complete the quest.
If the Vault Dweller returns to the leader of the underground ghouls after finding the junk, but before fixing the pump and talks to him, they will receive three skill books.
The quest can still be completed for 1000 XP if Necropolis has been attacked by the super mutants.