Fallout 1 -> Necropolis
Several quests, a wealth of readily available weapons, and a filter for Vault 13 await you in Necropolis . Upon entering, you'll be greeted by undead—people who suffered from radiation poisoning from a vault located beneath the city, but whose doors remained open. Go to the sewer hatch, open the cover, and descend.
Continue straight ahead, heading north. The city above is divided by garbage barricades, preventing you from reaching the other side of Necropolis from reaching the other side. After descending into the wastewater treatment plant, proceed forward, then left, then forward again to the exit to another location. There, you'll be greeted by a ghost shouting "don't shoot." Talk to him and learn about the water supply problem in Necropolis. Receive the quest to repair the wastewater treatment filter.
Next, go right and then up north again. After reaching another location, climb the stairs and find a hidden door in the wall. Disarm the trap and open the door, and you'll find yourself in front of the undead leader, Seth. He'll give you a task: eliminate the mutants in the water shed. Return down the same path and continue north without turning anywhere until you reach the last staircase leading up (you can turn right to a dead end where you'll see mole rats; kill them and find a plasma gun among the ghost corpses). Climb up and pass the ghosts, but they may attack.
You'll see Lemmy, a super mutant, and a hatch leading down next to him. Go down and head right to where the mole rats are. Kill them and pick up the debris—it contains parts for repairing the water pump. Next, return to Lemmy and now head to the small house to the north where the main super mutant, Harry, is standing. You can trick him and pass him by, convincing him you're a biorobot. You can kill him, thus completing Seth's quest. You can follow him to the military base, which will be difficult for an under-leveled player to escape from. There are three more super mutants there, but only the one with the flamethrower is dangerous.
Next, go to the water pump and use the junk parts on it. This will complete the quest to repair the pump in Necropolis. Next, go to the cell door and free the ghost; he'll tell you where the water comes from. Go through the door to the left of the cell and go down. You'll be met by two undead—shoot them without hesitation. Go forward to the elevator and take it to the third floor. This is Vault 12 under Necropolis. Now all that's left is to go to the working computer, grab the filter, and return to Vault 13 ...but the evil glowing undead won't let you escape this vault under Necropolis that easily...
Upon returning to the Vault, Overseer will tell you that you've saved everyone, but after reading the report, he'll send you to destroy the mutant overlord, the Master, and the base where they're generated. These will be the two main quests, and completing them will end the game. There will no longer be a time limit (as with the 150-day filter). If you have version 1.0, there will still be a 500-day limit for all quests, and that 500 days includes the 150 days spent finding the filter. You just need to install a patch to fix this.
Don't try to find the water thief separately, as this quest is unfinished, but after you give the filter to Overseer, it will automatically complete. So, first, you need to find the Master. You leave the Vault again... Now to the Brotherhood of Steel.
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