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.
The Vault Dweller then needs to approach the water pump and use their Repair skill to fix the pump. A successful skill check will fix the pump (a tool will help greatly). A critical failure will remove the junk from the Vault Dweller's inventory and deal 1 point of damage to them.
Then the Vault Dweller can rest easy that they have saved the ghouls from death. For a while, anyway.
An alternative way to get this quest is from Set (If you haven't killed the Master yet). If the Vault Dweller has already searched Vault 15 and are in possession of the "junk", Set will have his surly minion, Garret, fast travel the Vault Dweller to the water pump. The Vault Dweller is then forced to repair it at gun point (not that it may matter).
The game only checks if the player has any junk in their inventory, allowing them 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 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.
If the Vault Dweller does not have a decent Speech level or Sneak level, they cannot get past Harry without fighting him.