Dave Lizewski (AARON JOHNSON) is an average teenager who, like his best friends Marty (CLARK DUKE) and Todd (EVAN PETERS), wouldn't stand out in a crowd and thus is essentially invisible to the girl of his dreams, Katie Deauxma (LYNDSY FONESCA). Of course, Dave's comic book obsession probably doesn't help matters, although he doesn't let anyone know he's ordered his own superhero outfit.
With that, he decides to hit the streets as "Kick-Ass," but his first foray into crime-fighting results in him being knifed in the gut and then hit by a car. The latter not only results in him receiving various metal plates throughout his body, but also the inability to feel pain. That gives him new confidence for his next outing as a superhero, and while he gets beat up quite badly, he saves a victim and in the process becomes a viral Internet sensation after video of his deed goes online.
That doesn't sit well with drug kingpin Frank D'Amico (MARK STRONG) who thinks this masked figure is responsible for thwarting the efforts of his goons. When his men, including Big Joe (MICHAEL RISPOLI) and corrupt cop Det. Gigante (XANDER BERKELEY), can't find Kick-Ass, Frank's teenage son, Chris (CHRISTOPHER MINTZ-PLASSE) -- who wants to join his dad in the family business -- offers to pose as another superhero, Red Mist, and thus lure the fellow teen into a trap.
Little do the criminals know that the real thorn in their side is Damon Macready (NICOLAS CAGE), a one-time cop -- and former partner to Sgt. Marcus Williams (OMARI HARDWICK) -- whose superhero alter-ego is Big Daddy and who has an axe to grind with Frank. Unlike the teens, Damon is highly trained and possesses a large arsenal of powerful weapons, something he shares with his equally proficient 11-year-old daughter, Mindy (CHLOE GRACE MORETZ), a.k.a. Hit-Girl.
As Dave continues on his quest, little does he anticipate the ramifications his actions will have not only on himself, but also for others.