Once upon a time, there was a company called Interplay that made some of the best computer games ever. One of its most well known franchises was the Fallout series, which was an isometric RPG set in a post-nuclear apocalypse wasteland inspired by 1950s culture.
The Fallout games were really good, especially the second one. Even the demo for the first game, which was a self-contained adventure that was custom made for the purposes of the demo, was excellent. Not every developer will go to the trouble of making a demo with about five different endings and multiple paths. Plus, there were miniguns and you could shoot people in the crotch.
Unfortunately, owing to some complicated financial mis-planning, Interplay went out of business and it didn't look like the fans of the game would ever get to see a third game in what is arguably one of the most replayable and enjoyable franchises ever created.
That was, until another developer came along and decided to buy the rights to Fallout so that they could make more and more games in the series.
Some people were a bit upset at this news because while Interplay was a company that had shown the ability to make lots of different games with lots of different themes, the new company had only ever really made one type of game, over and over. The one type of game they had made was very good and they were very good at making it, but some people decided that a first-person sandbox RPG developer wouldn't fit very well with a tactical, isometric RPG franchise.
Later, those people were unveiled to be idiots and Fallout 3 was celebrated as perhaps one of the best games the new company had ever made. Way, way better than Oblivion for example, which always felt a little bit samey and empty after a little while.
A lot of this was down to the simple fact that the people at the new company aren't stupid. They know that they were best at making first-person RPGs and they know they have an engine that is good for making those types of games. They also knew that, in order to make Fallout 3 any good, they have to keep a lot of the same stuff from the first two games in it. So, that's what they did. Major fear allayed: Fallout 3 is still very Fallout-y.
The setting for the game is pretty much the same as the first one, though with a few more sinister twists in the opening and a more personal goal than “Hey, go get us that circuit board we need and do it quick!” Instead, you're looking for your father who has left the huge underground vault that has protected you since birth from the radiation-ravaged wasteland on the surface.
In any other Fallout game, leaving the vault wouldn't be that big a deal as the vaults were pre-programmed to open after a certain amount of time. As through players of Fallout 2 (and beginners in Fallout 3) can find out though, the vaults aren't all they seem to be. What is masquerading as a safe haven in a time of nuclear war is actually a vast social experiment, with each vault given certain conditions.
Vault 13, which was featured in Fallout 1, found itself low on water supplies sooner than it should have. Vault 12 was never able to seal properly and everyone inside was mutated. Vault 15 was full of political radicals. Vault 27 was deliberately overcrowded. Vault 42 had no light bulbs of more than 40 watts provided. Vault 77 had only one man inside and a crate full of puppets.
And Vault 101, your home and the place where Fallout 3 starts with your rather dramatic birth, was a vault that was designed to never re-open once it had shut. The occupants were trapped inside and caught under the thumb of a particularly over-zealous Overseer and the gang culture he allowed to form among the vault's youth. Until your father escaped, that is.