Fallout 3 is based on a futuristic world after the chinese have invaded, repelled by the US army, and then bombarded The USA with nukes. You were born in a vault(a fallout shelter) and raised there until your father leaves. with the vault's overseer trying to kill you for your "betrayal" you must follow your father and see why exactly he left in the 1st place. On your way you will encounter many citizens of the wastelands, whether they are enemies (Raiders, Super Mutants, Enclave, etc.) or friends (brotherhood of steel, friendly wastelanders, etc.). Side quests will pop up left and right giving u plenty of opportunities to gather weapons, experience, caps(treated as money), and information.