Dave Lizewski is an ordinary teenager who lives in Staten Island, New York. Inspired by comic books, Dave plans to become a real-life superhero. He purchases and modifies a scuba diving suit, and arms himself with batons. During his first outing, he gets stabbed and then hit by a car. After recovering, he gains a capacity to endure pain and enhanced durability due to having some bones replaced with metal.
In his absence from school, a rumor spreads that he is gay. As a result, his longtime crush, Katie Deauxma, immediately attempts to become his friend. Unhappy with the misunderstanding, Dave nevertheless appreciates the opportunity to get closer to Katie.
Dave returns to crime-fighting and gains notoriety after saving a man from a gang attack. Calling himself “Kick-Ass”, he sets up a Myspace account where he can be contacted for help. Responding to a request from Katie, he confronts a drug dealer, Rasul, who has been harassing her. At Rasul’s place, Kick-Ass is quickly overwhelmed by Rasul’s thugs. Before they can kill him, two costumed vigilantes, Hit-Girl and her father, Big Daddy, intervene, easily slaughter the thugs and leave with their money. After coming home, Dave realizes he is in over his head, and plans to give up crime-fighting. However, Hit-Girl and Big Daddy pay him a visit and encourage him.
Big Daddy’s real identity is Damon Macready, formerly an honest cop. Framed by Mafia boss Frank D’Amico, he was jailed. His wife committed suicide, leaving behind his daughter Mindy. Against the protest of his former partner Marcus Williams, Damon trains himself and Mindy as preparation for getting revenge on Frank. They have been undermining Frank’s operations by raiding his warehouses, robbing his money and destroying his drugs.
Frank believes Kick-Ass is responsible for the attacks and targets him, impulsively killing a party entertainer who is dressed like Kick-Ass. Frank’s son, Chris, suggests a different approach. He poses as a new vigilante, “Red Mist”, and befriends Kick-Ass. He plans to lure Kick-Ass into Frank’s lumber warehouse and unmask him. However, they find the warehouse on fire and Frank’s men dead. Red Mist retrieves a hidden camera he earlier placed in the warehouse, and sees recorded footage of Big Daddy killing the men and burning the warehouse. Red Mist and Kick-Ass part ways. D’Amico watches the footage and learns of Big Daddy and Hit-Girl.
Following the event, Dave decides to quit being Kick-Ass. He reveals his identity to Katie, and clears up the misunderstanding about him being gay. She forgives him and becomes his girlfriend. However, Red Mist contacts him again, and tricks him into revealing Big Daddy and Hit-Girl’s location. At one of Big Daddy’s safe houses, Red Mist shoots Hit-Girl out of a window, and Frank’s men capture Big Daddy and Kick-Ass.
Frank intends to have his thugs torture and execute his captives in a live Internet broadcast. While Kick-Ass and Big Daddy are being beaten by Frank’s gangsters, Hit-Girl, having survived the shooting, storms the hideout and kills all of the gangsters. During the fight, one thug sets Big Daddy on fire. Big Daddy and Mindy say a tearful farewell before he dies of his burns.
Kick-Ass and Hit-Girl resolve to defeat Frank D’Amico once and for all. Hit-Girl infiltrates Frank’s headquarters and kills numerous guards and henchmen before running out of bullets. When she is cornered by the thugs, Kick-Ass arrives on a jet pack fitted with miniguns and kills the remaining thugs. Kick-Ass and Hit-Girl then take on Frank and Red Mist. Kick-Ass fights Red Mist, which results in them knocking each other out. Frank overpowers an exhausted Hit-Girl. Before he can kill her, Kick-Ass regains consciousness and blasts Frank out of the window with a bazooka, killing him. Dave and Mindy retire from crime-fighting; Marcus becomes Mindy’s guardian, and she enrolls at Dave’s school.
Meanwhile, Chris D’Amico sits in his father’s office, dressed in an upgraded suit. Facing the camera, he says, “as a great man once said, wait’ll they get a load of me,” before firing a gun at the screen.