Arthur Poppington (Woody Harrelson), a slightly retarded guy, talks to a psychiatrist (Michelle Oh) and recounts his adventures as a costumed crime fighter called Defendor.
He's being held in a psychiatric ward for observation.
When he's in his costume he really thinks he's a superhero, though he often gets the living crap kicked out of him. He "rescues" a young woman (Kat Dennings of NICK & NORA'S INFINITE PLAYLIST) from a dangerous situation on the streets and they become unlikely friends. Arthur has it in his mind that a certain local criminal is responsible for the death of his mother (who was also a prostitute) and is determined to destroy his empire.
I believe this is supposed to be a comedy but I found it more sad and depressing than anything else. It's an interesting "real-life" take on someone being a superhero, much more like the movie SPECIAL than KICK-ASS.