The Earth was nearly wiped clean of life. A few managed to reach the relative safety of great underground vaults which remained sealed for a hundred and sixty years. But he was exiled by the rest of the vault people, for he had become something else in their eyes and no longer their champion. You are the descendant of that Vault Dweller. Your village is dying.
Crops are sick.
Brahmin are sick. Children are sick. The GECK is your only salvation. The game is extremely non-linear, and you have extreme flexibility. You can be a guardian angel doing good deeds and helping those in need.