The world has been destroyed and you've been safely living in Vault 101, one of many bomb shelters throughout the D.C. area. One day your father leaves the Vault and you follow, searching or answers and your missing father. Fallout 3 is an epic role-playing game with multiple paths and endings and incredible possibilities.
From the comments:
"Bethesda Wins Hands Down. Their Open World games (Oblivion and Fallout mainly) are very large, with much content and gameplay, not to mention Planned DLC for Fallout 3." -- Controller4