Dave Lizewski is an average high-school student with nothing special. He loves comic books, has got a few friends and lives alone with his father in New York. He spends his time with his frieds, Marty and Todd, who are also comic-book fands. He hasn't got a very difficult life and his challenges are just the challenges of a typical teenager.
However, one day, he makes a simple decision: to become a superhero, even if he hasn't got powers and he doesn't look like a superhero. He can't fly, can't fight and he can't run fast!
Dave is also in love with a girl, Katie, but she doesn't notice him. He doesn't exist for her.
One night, Dave helps a man who is attacked by three men in front of a cafeteria. Teenagers watch the fight between Dave and the men and record it with their mobile phone cameras. They put the video on the internet and Kick-Ass immediately becomes famous and Katie starts to notice him.
Meanwhile powerful gangster Frank D'Amico hates Kick-Ass and plans to destroy him...