Fallout 3 is set in the year of 2277, 36 years after the events of Fallout 2. The main setting is the Capital Wasteland, the remains of the city of Washington DC and it's surrounding area.
The main character, named the Lone Wanderer begins the game as a child and the player follows his early life through his home of Vault 101. Eventually, the Lone Wanderer's father makes a decision and leaves the safety of the Vault, forcing his child to fight his way out as the authorities try to intercept him. He escapes into the Capital Wasteland and begins his quest to find his father and inevitably becomes part of his fathers plan to bring clean water to the wasteland, making tremendous impacts on it's future.