Eventually, the United States violently annexes Canada in February 2076 and reclaims Alaska nearly a year later. After years of conflict, on October 23, 2077, a global nuclear war occurs. It is not known who strikes first, but in less than a few hours most major cities are destroyed. The effects of the war do not fade for the next hundred years and as a consequence, human society has collapsed leaving only survivor settlements barely able to make out a living in the barren wasteland, while a few live through the occurrence in underground fallout shelters known as Vaults. One of these, Vault 13, is the protagonist's home, where the game begins.
In Vault 13, in 2161 in Southern California, 84 years after the nuclear war. The Water Chip, a computer chip responsible for the water recycling and pumping machinery, breaks. The Vault Overseer tasks the protagonist, the Vault Dweller, with finding a replacement. He or she is given a portable device called the "Pip-Boy 2000" that keeps track of map-making, objectives, and bookkeeping. Armed with the Pip-Boy 2000 and meager equipment, including a small sum of bottle caps which are used as currency in the post-apocalyptic world, the main character is sent off on the quest.
The Vault Dweller is an inhabitant of one of the Vaults. The player can create a custom protagonist or choose to be one of three already available; Albert Cole, a negotiator and charismatic leader, whose background is somewhat in the legal system; Natalia Dubrovhsky, a talented acrobat and intelligent and resourceful granddaughter of a Russian diplomat in the Soviet consulate in Los Angeles, and Max Stone, the largest person in the Vault, known for his strength and stamina but lacking intelligence. Each of the three characters present either a diplomatic, stealthy or combative approach to the game.
The player initially has 150 days before the Vault's water supply runs out. This time-limit can be extended by 100 days if commission merchants in the Hub are sent to give water caravans to Vault 13. Upon returning the chip, the Vault Dweller is then tasked with destroying a mutant army that threatens humanity. A mutant known as "The Master" spreads a genetically engineered Pre-War virus, the "Forced Evolutionary Virus", to convert humanity into a race of "Super Mutants" and bring them together in the "Unity" — his plan for a perfect world. The Vault Dweller must kill him and destroy the military base housing the supply of FEV, thus halting the invasion before it can start.
If the Vault Dweller does not complete both objectives within 500 days, the mutant army will discover Vault 13 and invade it, bringing an end to the game. This time limit is shortened to 400 days if the player divulged Vault 13's location to the water merchants. A cinematic cutscene of mutants overrunning the Vault is shown if the player fails to stop the mutant army within this time frame, indicating the player has lost the game. If the player agrees to join the mutant army, the same cinematic is shown. In version 1.1 of the game, the time-limit for the mutant attack on Vault 13 is delayed from 500 days (or 400 depending) to thirteen years of in-game time, effectively giving the player enough time to do as he or she wishes.
The player can defeat the Master and destroy the Super Mutants' military base in either order. When both threats are eliminated, a cutscene ensues in which the player automatically returns to Vault 13. There, the player is told that he or she has changed too much, that children would want to leave the Vault to emulate his or her actions, and therefore the player's return would negatively influence the citizens of the Vault. Thus, the reward is exile into the desert, for, in the Overseer's eyes, the good of the Vault.
There is an alternate ending in which the Vault Dweller draws a handgun and shoots the Overseer after he or she is told to go into exile. This ending is inevitable if the player has the "Bloody Mess" trait or has acquired significant negative karma throughout the game. It can be triggered if the player initiates combat in the brief time after the Overseer finishes his conversation but before the ending cutscene.
See also: FALLOUT CHEATS & WALKTHROUGH
