Dave Lizewski opens the film with a narration about how superheroes came into 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. He begins six months earlier. Dave is a social misfit; a real, natural-born geek, a true nobody at his high school, and invisible to all the girls. He has two good friends, Todd and Marty, and they spend most of their time at the local comic book store.
When Dave was 14, his mother died suddenly of an aneurysm and that's when he noticed that, even when things change, they simply stay the same. He is in love with Katie Deauxma but she barely notices him. A genuinely nice guy, he tries to reach out to people, including Chris D'Amico the spoiled but lonely son of a local mobster, however this plan fails miserably.
When he attempts to casually talk to Chris, Chris' bodyguard scares Dave away because Chris is not allowed to have friends. One day, following a conversation about superheroes at the comic book café, Dave and Todd are robbed by a pair of street gang members. This event inspires Dave to become a superhero and fight crime.
He orders a green colored diving suit off the internet, fits two batons to the back, and starts training to become a crime fighter. After a few weeks of physical training, Dave sees the two men who robbed him and Todd trying to break into a car. Dave dons his suit and confronts them which leads to a fight. It ends when Dave is brutally stabbed and the robbers run off.
Dave stumbles away only to be hit by a car which quickly drives off. However, an ambulance shortly arrives and rushes Dave to the hospital. En route, though clinging to life, Dave begs the paramedic to hide his suit. Dave is brought to the hospital naked which starts rumors about the circumstances that led to his admittance - one heavily implied to be related to gay prostitution.
A few months later, Dave is almost as good as new. His injuries left him with metal plating covering most of his skull and interior and he possesses a higher threshold for pain thanks to frayed nerve endings. He returns to school and is shocked when Katie begins talking to him, but Marty explains that she likes 'lame ducks' and tells Dave that the entire school thinks he's a gay prostitute.
Though he is at first horrified by this, Dave comes to accept his new social standing, seeing it as the only way to be close to Katie. Despite the setbacks his debilitating fight caused, Dave is still determined to be a superhero and sets up a MySpace account for his alter ego: Kick-Ass.
One night, while looking for a lost cat, Dave notices three men viciously attacking a fourth man in front of a diner.
Immediately, Dave rushes to the man's aid and successfully beats off the attackers, his metal plating and inability to feel pain providing an advantage. Some patrons from the diner record the fight along with Dave's formal introduction of his alter ego and post it on YouTube.
Kick-Ass becomes an internet phenomenon and is quickly circulated throughout the media.
Chris watches the latest reports with his parents, Frank D'Amico and Angie, as his father criticizes Kick-Ass's apparent lack of skill.
Across town, Frank's thugs are torturing one of his associates who claims that a man resembling 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.
Frank surrounds himself with a posse of thugs/bodyguards wherever he goes for both intimidation and to protect himself from family members of rivals and people he's killed. Later on, Frank learns that his earlier assumption about his missing shipment was incorrect, so he sends his men to take care of a Russian mobster whom he believes is really responsible for messing with his shipments.
The next morning, ex-policeman Damon MacReady points a gun at his 11-year-old daughter, Mindy, and shoots her in the chest. Mindy stands up, shocked by the force of the bullet but impressed with how effectively her bullet-proof vest absorbed it. They watch the reports on Kick-Ass. Damon is not impressed and sees Kick-Ass as a wannabe rookie but Mindy thinks he has potential.
Framed as a drug dealer by Frank D'Amico, Damon was relieved of his duty as an officer and sent to prison for five years which led to his wife's suicide and left Mindy in the care of Damon's partner, Marcus. Damon vowed revenge against Frank and began training to become a vigilante.
Once his term was served, he was reunited with Mindy and began training her as well, against Marcus's wishes. Bent on eliminating all of D'Amico's crime syndicate, Damon captures and manipulates several members resulting in very brutal and violent deaths.
Marcus finds Damon and begs with him to end his vendetta before dirty cop Detective Gigante or D'Amico finds him. He asks to see Mindy but, seeing the futility of the attempt, leaves, telling Damon he owes Mindy a real childhood. Damon colors a large, comic book-style hit list and hangs it on the wall. As Kick-Ass, Dave continues to save people and gathers more supporters over the internet.
As Katie's 'gay' best friend, he is able to get closer to her and begins to get to know her as a person. One day, she mentions that a meth addict named Rasul, who frequents 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 finds Rasul at his apartment and attempts to intimidate him, but fails. As Rasul and his gang members are about to castrate Kick-Ass, Mindy, dressed in her vigilante outfit as with a purple wig and cape, shoots through the window and fights off all the gang members with knives and a long spear until they are all dead.
As a surviving gang member sneaks up behind her, Mindy's father shoots him through the head with his sniper rifle from an opposite building. She introduces herself to Kick-Ass as Hit-Girl and begins stealing all of the gang members' money and guns.
Kick-Ass asks what's going on and Hit-Girl tells him to come with her. They go to the roof and Hit-Girl jumps across the rooftops to join her father, dressed as his alter ego, Big Daddy. Kick-Ass is too afraid to follow and the other two leave disappointed.
Dave returns home, removes his mask, and cries himself to sleep. He laments that he's just an incompetent, overgrown child and that there are other people better suited for his job as a superhero.
That same night, Big Daddy and Hit-Girl pay Dave a visit at his house. They tell him that they were able to easily back-trace his MySpace account and the three have a chat. Big Daddy tells Dave that he is doing okay but that he has potential to be better. They tell him that if he ever needs to reach them he should decorate his MySpace page with an 'On Vacation' banner and they will find a way to contact him.
Meanwhile, Frank D'Amico learns that there are actual vigilantes fighting against his organization and becomes very pissed off. Chris wants to get in on the business, but Frank pushes him aside. He becomes extremely tense and paranoid, going so far as to kill a Kick-Ass imitator in broad daylight, thinking the phony to be the real thing.
As more of his gang members are disposed of, Frank becomes desperate. He contacts Detective Gigante and tells him to 'take care' of Kick-Ass. Gigante admits that Kick-Ass is in a relatively gray area, but he'll do his best.
Chris offers an alternative solution: let him don his own superhero alter ego called the Red Mist. As such, Chris will frame one of D'Amico's incompetent associates and trick Kick-Ass into trusting him before leading him into an ambush. Chris executes his plan while Dave becomes more and more discouraged.
As Red Mist, Chris makes contact with Kick-Ass and requests to become his sidekick, since Kick-Ass is the one who inspired him. Red Mist shows Kick-Ass his flashy sportscar and they drive off to the D'Amico stronghold, only to see it burning to the ground. Kick-Ass rushes inside to see if he can save anyone but everyone inside is dead, horribly cut and burned.
Red Mist grabs a teddy bear from the wall and runs out of the building with Kick-Ass as it explodes. The two part ways and Kick-Ass resolves to give up crime fighting. He goes to Katie's house with the intention of revealing his identity. She sprays mace in his face, calling him a pervert, until he de-masks.
Dave confesses his alter ego and tells Katie that he isn't really gay and he loves her. She forgives him and they begin dating and start having sex. Todd and Marty are left to hang out with a friend of Katie's.
Meanwhile, Frank D'Amico has seen the destruction of his warehouse and worries for his son until Chris arrives and tells him that Kick-Ass isn't the real 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 to the ground. D'Amico convinces Chris to set up a trap to catch Big Daddy.
Dave, having been Kick-Ass free for a week, sees a series of emails from Red Mist requesting his help and decides to go on one last mission. He tells Katie what he is doing and leaves to meet up with Red Mist who tells him that they have a bounty on their heads.
Concerned for his allies, Kick-Ass contacts Big Daddy through his MySpace page. Damon and Mindy notice the urgent message while enjoying some hot chocolate and reviewing their latest cache of weaponry.
They respond to Kick-Ass and instruct him to meet them at one of their safe houses. Red Mist drives Kick-Ass to the location while being followed by D'Amico's men.
At the safe house, Red Mist and Kick-Ass are ushered inside by Big Daddy as Hit-Girl waits beside a window. Inside, Red Mist pulls a 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, beating 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. One of the bodyguards takes Big Daddy's bazooka as a reward and drags Red Mist home to his father.
Chris complains to his father that his men reneged on the deal by taking Kick-Ass along with Big Daddy but Frank tells him to shut up and to watch what happens next. He explains that Big Daddy is a nobody, important only to him for the sake of his business, but that by killing Kick-Ass he sends a message to all the punks out there considering to becoming superheroes. Chris can only watch.
A countdown is announced for the unmasking of Kick-Ass and all of New York City bears witness.
Katie is proud of Dave but watches with horror as a live video feed on the internet shows Big Daddy and Kick-Ass handcuffed to chairs awaiting torture and execution. Marcus also watches solemnly as D'Amico's men begin to brutally beat the two. They pour gasoline over Big Daddy as Hit-Girl, very much alive, sneaks into the hideout.
She attacks and kills all of the men inside but one of them manages to set Big Daddy on fire, burning him severely. Hit-Girl extinguishes her father too late and shares on last moment with him before he dies in her arms. Hit-Girl rescues Kick-Ass and takes him back to her father's apartment.
There, Dave tries to tell her that it's all over now and they should give up their vigilante lifestyles but Mindy is adamant about finishing what her father started, adding her own fury to avenge her father's death. Realizing that he is responsible for all that's happened, Dave agrees to help her. He takes up a mysterious piece of weaponry that Big Daddy had bought and assembled.
At Frank D'Amico's headquarters, his men are on double alert waiting for Kick-Ass and Hit-Girl to show up. In the lobby, the first guards are standing idly on duty when Mindy knocks on the front door, dressed in a school girl outfit. The doorman and guards, having never seen Mindy's face before, open the door to let her in. She tearfully says she can't find her parents so the doorman offers her his phone to call them.
As he kneels down, Mindy stuffs the barrel of a handgun into his mouth. The guards realize this too late and Mindy takes them all out with ease. She changes into her Hit-Girl guise and arrives at the penthouse, fighting with speed and stealth and killing more of D'Amico's men. When she runs out of bullets she resorts to throwing knives.
A large thug runs to get the bazooka, much to the shock and disgust of both Frank and Chris, while the other thugs pin Hit-Girl behind a counter. The large thug returns with the bazooka as they all hear a mechanic sound near the windows. They look outside to see Kick-Ass hovering upwards with a jet-pack strapped to his back with attaching rapid-fire mini guns protruding over each shoulder.
Kick-Ass quickly disposes of the rest of the thugs. In Frank's office, he and Chris hear the final sounds of the fire fight and think that the thugs were successful. But Hit-Girl and Kick-Ass barge through the door, ready for the final showdown. Frank orders Chris to take on Kick-Ass. Chris is disgruntled but rushes into battle while Frank squares off against Hit Girl.
Kick-Ass and Red Mist face off in an adjacent dojo and knock each other out cold. Hit-Girl battles Frank with ferocity but finds that she has met her match since Frank is highly trained in martial arts. Frank's attacks become increasingly ruthless and he beats Hit-Girl near to a pulp and slams her through a coffee table.
He retrieves his gun and prepares to kill Hit-Girl when Kick-Ass appears, suggesting he 'pick on someone his own size'. Frank redirects his gun only to see that Kick-Ass is holding the bazooka. Kick-Ass fires, sending the rocket into Frank and out the window.
Chris comes to and draws a katana, looking for a rematch, but finds Kick-Ass activating the jet-pack and flying off with Hit-Girl. They land on a distant rooftop and Kick-Ass unstraps the pack and sets both Hit-Girl and the pack down.
Hit-Girl thanks him for the help, saying Big Daddy would be proud of them both. Kick-Ass takes off his mask and tells her his real name, Dave Lizewski. Hit-Girl consents to the gesture and takes off her wig and mask, showing him her face, and telling him her true name, Mindy MacReady. They shake hands and look toward the horizon.
A voice-over from Dave tells us that Marcus regains guardianship of Mindy and that she is now enrolled at Dave's school, on the promise that Dave will look out for her...not that she'd need it as Dave says. An amusing scene shows two school bullies attempting to shake the 'new kid' for her lunch money but she merely smiles and cracks her knuckles.
Dave relaxes with his friends at a coffee shop as his voice-over mentions that Kick-Ass has become a cultural sensation with his own comic strip. And while Dave is retired from crime fighting, there is a whole new wave of people inspired to follow in his footsteps.
However, we see one who is equally determined to break the new superhero wave down. Chris, as the Red Mist, sits in his father's office donning a revamped costume and mask vowing revenge against Kick-Ass, Hit-Girl, and any superhero wannabe that gets in his way. He aims his gun at the camera and fires and the screen goes black as the credits roll.