David Dunn, security guard of a rugby stadium, is the sole survivor of a terrible train crash. Everybody else dies, but he survives without a scratch.
When he attends the funeral of the casualties, he finds a letter on his windshield, asking how many days of his life has he been sick. David discovers that the answer is "none", and contacts the sender of the letter, Elijah Price, the owner of a No. 1 comic book antiques store. Elijah has a serious illness, his bones are very fragile, and he wants to prove David his theory: that if there is him, a highly "breakable" man, there must be the opposite, the "unbreakable", a superhero.
David has never been sick or wounded in all his life; he can lift unimaginable weights; and he has certain superheroic powers, for example to learn people's thoughts by merely touching them. David must have a weakness too, like Superman had - it turns out he does have one, and that weakness is water. He almost drowned once when he was a kid. At this point, David starts to believe in Elijah's theory, and he is off to save the innocent.