If you dont know the story, i'll give a outlook.
Fallout 3 is set in a post-apocalyptic environment set in Washington DC and the surrounding area. basically, the events around world war II did not happen in the fallout universe. Life went on naturally, but set in a 1950's style for several decades until the late 21st century. The United States and the Chinese went to nuclear war, destroying much of the planet and breeding the current environment of genetic mutation, strange creatures and radiation poisoning all over the shop.
Your character is born into "vault 101". Vaults are small inhabitation centers set up by Vault Tec. the inhabitants are protected from the outside environment, but are unable to leave (due to rules and regulations). The rules of the administrator are law. You escape after your father leaves the vault. so you have to go out and survive in the Capital Wasteland, the area that used to be Washington DC.
Its sorta like a futuristic post apocalyptic environment, thats based off of the Oblivion style of play.
