There are heroes, there are superheroes, and there is Hancock. Possessing superpowers implies responsibility, everyone knows that - except him. He takes on every task with heart and the best of intentions, saving lives - at the cost of inhuman destruction and incalculable damage. In the end, the public's patience comes to an end: people are grateful to their local hero, but sometimes they don't understand why they deserve such punishment.
Hancock isn't the kind of guy who cares about public opinion, but one day, after another feat of saving Ray's high-ranking PR man, he suddenly realizes that in some ways he may be vulnerable. This is hard to accept, especially according to Mary, Ray's wife, who thinks that Hancock is simply hopeless.