Dave Lizewski (Aaron Johnson) is an ordinary teenager who lives in Staten Island, New York. Dave purchases a bodysuit and makes modifications to it. He then embarks on a campaign to become a real-life superhero, armed with a couple of batons. During his first attempt to fight crime, he becomes the victim of a stabbing and a hit and run. This leaves him with multiple fractures and metal plates, all of which cause him to require extensive rehabilitation. In the process of recovery, he gains a capacity to endure pain and enhanced durability. In his absence from school, a rumor spreads that he is gay. (Due to him having gotten rid of his costume and being found naked by the paramedics). As a result, his longtime crush, Katie Deauxma (Lyndsy Fonseca), immediately attempts to become his friend.
Dave returns to crime-fighting and gains notoriety after intervening in a gang attack. Calling himself "Kick-Ass", he sets up a Myspace account where he can be contacted for help. After responding to a request from Katie, he deals with a drug dealer, Rasul (Kofi Natei), who has been harassing her. Kick-Ass uses a taser against Rasul, but in the event, his thugs quickly overpower him. To his shock he is rescued by an eleven-year-old vigilante Hit-Girl (Chloë Grace Moretz), who slaughters Rasul and all of the thugs and leaves with her father, Big Daddy (Nicolas Cage).
Big Daddy is Damon Macready, an honest former cop who was framed as a drug dealer by Mafia boss Frank D'Amico (Mark Strong), which led to him being jailed and his wife to commit suicide, after giving birth to Mindy. Against the advice of his former partner Marcus (Omari Hardwick), Big Daddy trains his daughter Mindy to be a skilled crime-fighter and works to undermine D'Amico's operations. However D'Amico believes Kick-Ass is responsible for the attacks and targets him, impulsively killing a party entertainer dressed like Kick-Ass. D'Amico's son, Chris (Christopher Mintz-Plasse), suggests a different approach. He assumes the role of the vigilante "Red Mist" in order to befriend Kick-Ass and lure him into a trap at D'Amico's lumber warehouse. When Kick-Ass and Red Mist arrive at the warehouse, Frank's men are dead and the warehouse is on fire. With the help of Chris's teddy cam, they find out it was Big Daddy who killed Frank's men and burnt down the warehouse.
Following his lucky escape, Dave decides to quit being Kick-Ass. He confesses the truth to Katie about being Kick-Ass (and not being gay), and she forgives him and becomes his girlfriend. After finding a number of messages from Red Mist urgently requesting they meet, Dave decides to don his Kick-Ass costume one last time. At their meeting, Red Mist convinces Kick-Ass to call his allies, unwittingly luring Big Daddy and Hit-Girl into an ambush. Upon arriving at one of Big Daddy's safe houses, Red Mist shoots Hit-Girl out of a window, and D'Amico's men storm the place, capturing Big Daddy and Kick-Ass. During the commotion, one of Frank's bodyguards gleefully steals a bazooka that is mounted on the wall. Frank intends to have his thugs torture and execute his captives in a live Internet broadcast viewed by millions. While Kick-Ass and Big Daddy are being brutally beaten by Frank's gangsters, Hit-Girl, having survived the shooting, storms the hideout and kills all of the gangsters. During the struggle, one thug sets Big Daddy on fire. Hit-Girl and Big Daddy say a tearful farewell before he dies of his injuries.
Hit-Girl at first shows anger towards Kick-Ass for her father's death but cools down when Kick-Ass, out of guilt decides to help Hit-Girl defeat Frank D'Amico once and for all. Later, Hit-Girl infiltrates Frank D'Amico's headquarters, killing numerous guards and henchman on her way. During the battle in the penthouse, she runs out of bullets. Before the thug who stole the bazooka can fire at her, Kick-Ass arrives on a jet pack fitted with miniguns that Big Daddy had previously acquired, and kills the remaining thugs. Kick-Ass and Hit-Girl then take on D'Amico and his son. While Kick-Ass fights Red Mist in the training room and they knock each other out, Hit-Girl is overpowered by Frank. As Frank aims his gun and prepares to finish off Hit-Girl, Kick-Ass regains consciousness and comes to the rescue armed with the bazooka. Frank is blasted out of the window where he explodes in mid-air.
Red Mist regains consciousness and grabs a samurai sword but is too late and arrives in time to see Kick-Ass and Hit-Girl leaving on the jet pack. Mindy and Dave retire from crime fighting to live a more normal life, and Mindy goes to live with Marcus and enrolls at Dave's school. Dave has been told to look after Mindy, "not that she needs it". Dave is done with crime fighting, happy to have inspired a new "generation" of superheroes. Chris, still wearing his Red Mist outfit, is shown donning a new mask as he quotes Jack Nicholson as the Joker, "As a great man once said, 'Wait till they get a load of me'".