The storyline of "Kick-Ass" has to do with a teenage boy, Dave Lizewski (Aaron Johnson), who decides to turn himself into a bona fide superhero after being motivated by the heroes of comic books.

Lizewski blunders in his initial attempt to combat crime, winding up severely wounded in the process (which results in a brief time spent in a hospital following his being stabbed).

Dave is somehow undaunted by this minor setback, and winds up getting in the middle of a brawl outside a cafe. He fights them with two modified night sticks and wins. A male patron in the diner, who filmed the occurrences on his cell phone, asks Dave his identity, and Dave tells him: "I'm Kick-Ass". The video goes viral and becomes an Internet phenomenon, and "Kick-Ass" becomes an overnight star.

His efforts at being a super hero very quickly lead him to realize it isn't as glamorous as it appears. Soon he meets a strange young vigilante called Hit-Girl (Chloe Moretz), and her father Big Daddy (Nicolas Cage). They were already aware of the intrepidity of Kick-Ass through YouTube and the media.

This resulted in them joining together to work on getting rid of the town crime syndicate boss Frank D'Amico (Mark Strong). Throughout the journey, Kick-Ass becomes acquainted with another hero, Red Mist (Christopher Mintz-Plasse), who admires Kick-Ass's adventures.