Dave Lizewski (Aaron Johnson) opens the film with a narration about how superheroes come to existence. A man in a superhero costume stands on a skyscraper above a crowd ready to show the world his new flight suit. He jumps as people cheer before crashing into a taxi and dying. Dave comments that the man isn’t him, just a delusional mental patient. He introduces himself as Kick Ass and starts telling the story of his life.
Dave is a nobody at his high school and invisible to all the girls. He has two good friends, Toddy and Marty, and spends his time at the local comic book store. His mother suddenly died of an aneurism when Dave was 14 and he noticed that even when things change, they simply stay the same. He is in love with Katie Deauxma (Lyndsy Fonseca) but she barely notices him. A genuine nice guy, he tries to reach out to people, including Chris D’Amico (Christopher Mintz-Plasse) the spoiled but lonely son of a local mobster but fails miserably. One day, after a conversation about superheroes at the local comic book café, Dave and Toddy are robbed and Dave decides that he is going to be a superhero and fight crime. He orders a Diving wet suit off the internet and starts “training” to become a real crime fighter.
After a few weeks of training, Dave sees the men who robbed him and Todd trying to break into a car so Dave puts on his suit and confronts them. He tries to fight them, but the fight ends with Dave being brutally stabbed. He stumbles around as the thugs run away, only to be hit by a car, which drives off as fast as it can. Clinging to life in the back of an ambulance, he is rushed to the hospital but begs the paramedic to hide his suit. He is brought to the hospital naked, which starts rumors about what happened to him before his accident (heavily implied by others to be gay prostitution).
A few months later, Dave is almost as good as new. He is full of metal plates and has numerous frayed nerve endings (granting him a higher pain threshold). He returns to school and is shocked that Katie is talking to him. Marty explains that Katie likes “lame ducks” and explains that the entire school thinks that Dave is a gay prostitute. Dave is horrified but comes to accept that this would be the only way to get closer to Katie. Despite these setbacks, Dave is still determined to be a superhero and sets up a MySpace account so that people can contact him and give him cases. One night, while looking for a cat, Dave fights off three attackers who were beating a defenseless man in front of a diner. Some patrons record the fight and post it on YouTube. Dave’s alter ego, Kick Ass, becomes an internet phenomenon.
Across town, Frank D’Amico (Mark Strong)’s thugs are torturing one of his associates who claims that a man who looks like Batman attacked their cocaine dealers and stole all of their drugs. Thinking it far more likely that the associate stole the cocaine for a profit, Frank has him tortured and killed before taking his son Chris out to the movies. However, he later learns that the information given to him was poor, so he sends his men to take care the Russian he believes is responsible for messing with his shipments.
Across town, ex-policeman Damon Macready (Nicholas Cage) points a gun at his daughter, Mindy (Chloe Moretz) and shoots her in the chest. Mindy stands up and is shocked by the force of the bullet on her armor. He is training her to be a vigilante to fight D’Amico’s minions in his personal vendetta. The two see Kick Ass’s exploits: Damon is not impressed, but Mindy thinks that he has potential. He was sent to prison for five years, during which Mindy was raised by his former partner, Marcus. Because his wife died during his prison stint, Damon has made it his goal to avenge her by killing D’Amico’s entire organization. He captures and manipulates several key members of D’Amico’s crime syndicate to their (very brutal) deaths. Marcus (Omari Hardwick) finds Damon and tells him that he has to end his vendetta before Detective Gigante (Xander Berkeley) finds them or D’Amico does. He begs Damon to let him see Mindy, but realizing the futility of the attempt, he leaves telling Damon that he owes Mindy a real childhood. Damon colors a large of comic book style hit list and hangs it on the wall.
As Kick Ass, Dave goes around saving people and gathering more supporters. As Katie’s gay best friend, he gets closer to her and begins to get to know her as a person. One day, she mentions that a meth addict named Rasul at the clinic she volunteers at robbed her and smacked her around, refusing to leave her alone. Dave convinces her to message Kick Ass. That night, Kick Ass hunts down Rasul at his apartment. He attempts to intimidate him but fails miserably. Right as Rasul and his gang members are about to castrate Kick Ass, and Mindy shoots through the window and fights all of the gang members until they are all dead. As the last gang member sneaks up behind her, Big Daddy (Damon) shoots him through the head with his sniper rifle from a different building. She introduces herself as Hit Girl and begins stealing all of the gang members’ money and guns. Kick Ass asks what is going on, Hit Girl tells him to come with her. They go to the roof and Hit Girl jumps across the rooftops to join her father. Kick Ass is too afraid to follow and the other two leave disappointed. Dave takes off his mask at home and cries himself to sleep, realizing that he’s just a child and that there are other people better suited for his job.
A few nights later Big Daddy and Hit Girl pay Dave a visit at his house. They tell him that they were able to back trace his MySpace account and the three have a chat. Big Daddy tells him that he is doing ok, and that he has potential to be better. They tell him that if he ever needs to reach them he should decorate his MySpace with an “On Vacation” banner, and that they will find a way to reach him.
Frank D’Amico learns that there are actual vigilante’s going against his organization and gets pissed. Chris wants to get in on the business, but Frank just pushes him aside. Assuming that Kick Ass is the perpetrator, Frank kills an imitator in broad daylight thinking that it was the real Kick Ass. As things become tenser and more of his gang members are killed, Frank becomes willing to try anything. He contacts Gigante and tells him to take care of Kick Ass. Gigante tells him that Kick Ass is in the morally gray area but that he’ll do his best. Chris offers an alternative suggestion: Let him don the identity of a “superhero” called the Red Mist, frame one of D’Amico’s incompetent associates and trick Kick Ass into trusting him while leading him into an ambush.
Chris executes his plan as Dave becomes more and more discouraged. He meets up with Red Mist and the two talk. Red Mist shows him his car and offers to be his side kick since Kick Ass “inspired” him. The two drive around and Red Mist takes him to the D’Amico stronghold, only to see it burning to the ground. Kick Ass is unaware that it was an ambush, but the two go into the building to see if they can save anyone. The bodies are horribly cut up and everyone is dead. Red Mist grabs a teddy bear on a wall and runs out of the building with Kick Ass as it explodes. The two part ways and Kick Ass resolves to give up the crime fighting business.
Kick Ass goes to Katie’s house to reveal his identity to her. She sprays him in the face and calls him a pervert until he de-masks. He tells that he isn’t gay, that he’s Kick Ass and that he loves her. He offers to go but she asks him to stay. The two start going out and having sex while Marty and Todd hang out with Katie’s friend.
Frank D’Amico sees that the stronghold has burned down and is worried about his son. Chris returns and tells Frank that Kick Ass isn’t the threat. He shows Frank the teddy bear, which has a Nanny Cam in it. They watch footage of Big Daddy killing all of D’Amico’s men and burning the warehouse down. D’Amico realizes that this is a serious problem and convinces Chris to set up a trap to catch Big Daddy. Red Mist tries to contact Kick Ass and decides to set the plan in motion.
Dave, who had given up on being Kick Ass for a week, sees Red Mist’s emails and decides to go on one last mission for him. He tells Katie what he’s doing and leaves. He and Red Mist meet up and set up the MySpace to contact Big Daddy. Big Daddy and Hit Girl are having hot chocolate and looking at their latest piece of purchased weaponry when they see the message. They send Kick Ass the meeting house location and Red Mist drives them there. As each hero gets ready for the meeting, the tension mounts. Red Mist drives Kick Ass to the location while D’Amico’s men follow the van.
Big Daddy opens the door as Hit Girl is perched on a window. He invites Kick Ass and Red Mist into the building when Red Mist pulls out his gun and shoots Hit Girl twice in the chest. As she falls out of the window, Big Daddy screams and D’Amico’s men rush in and beat him and Kick Ass unconscious. As they put them in the back of a waiting van, Red Mist screams for them to let Kick Ass go since he considers him a friend and doesn’t want to see him hurt. Chris’s bodyguard takes Big Daddy’s bazooka as a reward and drags Chris home to his father. Chris complains about how Frank’s men reneged on the deal, but Frank tells Chris to shut up and watch what happens next. He explains that Big Daddy is a no one…but that killing Kick Ass sends a message to all the punks considering becoming super heroes. Chris watches on in horror.
The city of New York is shown a countdown for Kick Ass’s unmasking. Katie is shocked but happy for Dave and watches the countdown. She’s horrified when it shows Big Daddy and Kick Ass handcuffed to chairs about to be executed. D’Amico’s men begin beating the pair brutally and stream it all online. They pour gasoline on Big Daddy and ignite him just as Hit Girl arrives to save them. She kills all the men and extinguishes her father but it’s too late. Big Daddy dies and tells Hit Girl that he loves her. Hit Girl saves Kick Ass and takes him to her father’s old apartment. Dave tells her that it’s over, but in a moment of self-reflection realizes that he is responsible. He agrees to help Hit Girl exact her revenge and takes up the mysterious piece of weaponry Big Daddy had bought and put together before his death.
Hit Girl infiltrates the D’Amico stronghold and shoots all the guards she can find until she reaches the top level. She kills most of the men, but she runs out of bullets, leaving three remaining henchmen. After using kitchen knives to kill one, Chris’s bodyguard goes to get the bazooka. Just as he’s about to destroy the area of the house with Hit Girl in it, Kick Ass arrives in Big Daddy’s jetpack Gatling gun. He kills the remaining henchmen and he and Hit Girl go to take care of the D’Amicos.
Hit Girl and Frank D’Amico fight in what’s left of Frank’s office while Kick Ass goes head to head with Red Mist. Kick Ass and Red Mist fight using his batons and they both knock each other out simultaneously. D’Amico beats Hit Girl to a pulp and pulls out a gun to kill her. Just as he’s about to blow her brains out, Kick Ass stirs and grabs the bazooka. He tells D’Amico to pick on someone his own size and fires the bazooka. D’Amico is shot out into the city and explodes. As Red Mist gets up and grabs a sword to kill Kick Ass with, he sees Kick Ass pick up Hit Girl and fly away. Red Mist is left wondering what happened to his father but knowing that he’s dead.
Hit Girl and Kick Ass land on a nearby building at dawn. They tell each other their real names and part. Mindy is adopted by Marcus and goes to the same school as Dave. Dave and Katie continue to go out. Meanwhile, Red Mist is left contemplating what he’s going to do and decides to become a super villain and avenge his father. He fires his gun at the screen and it cuts to black.
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