Dave Lizewski (Aaron Johnson) is an average teenager who wonders why no one has ever agreed to a super hero in the comic books become decision a superhero in real life itself, even without super powers or training of any kind. After its first attempt at crime Dave was beaten and stabbed to fight and hit by a car. He convinces paramedics not to mention his habit, and spent several weeks in the hospital. Come back to school, Dave is his old crush Katie (Lyndsy Fonseca), is surprisingly interested in him, but only because he thinks it is gay. Dave goes along with this, in an effort to spend time with her.

Undeterred by his first setback, Dave defends a man with three strikers. A passer-register the event as Dave who calls himself "Kick-Ass". The video became a YouTube phenomenon, Dave and create a MySpace account so people can contact Kick-Ass.

Katie admits that Dave was harassed by drug dealers yeah, he convinces her to seek help Kick-Ass. Kick-Ass tracking drug dealers to deliver a warning, but is attacked. He is then saved as a child-costumed vigilante Hit Girl (Chloe Moretz), who kills his attackers and then leave with his father, Big Daddy (Nicolas Cage). After tracing Kick-Ass, to inform them that they can work together and stay in touch.

Big Daddy is revealed Damon Macready, a former cop who was framed by local crime syndicate leader Frank D'Amico (Mark Strong). While Damon was in prison, his wife dies giving birth to Mindy. After his release Mindy Damon trains to fight crime as Hit-Girl.

Kick-Ass believe responsible for several deaths and the theft of drug money in his organization, D'Amico ordered his men to find him and eliminate. After D'Amico kills an impostor dress Kick-Ass, his son, Chris (Christopher Mintz-Plasse), suggests a different approach. Chris will be a superhero called Red Mist kick-ass confidence in him and trick him into a trap. Red Mist meets Kick-Ass, allowing the stock to D'Amico ambush "are men. Instead, they found in the burning building to die. Red Mist retrieve surveillance images showing that Big Daddy is responsible.

Realizing the gravity of his situation, Dave wants to stop Kick-Ass. Katie tells the truth, and she forgives him and his girlfriend. Some time later, Dave is urgent messages by Red Mist, requesting they meet, and find Hit Girl and Big Daddy. Kick-Ass Red Fog leads to one of the safe houses of Macready, D'Amico to the men below. Hit-Girl shoots Red Mist, knocking out a window. Kick-Ass and Big Daddy are captured and taken to a warehouse being tortured in a live broadcast. Kick-Ass and Big Daddy were beaten, and Big Daddy is on fire, then Hit Girl, survived the shot wore a bulletproof vest, comes in and killing the gangsters. Big Daddy is severely burned and died from his injuries after a tender moment with his daughter. Kick-Ass Hit-Girl tries to convince you to stop, but is expected to finish what his father had started and Kick-Ass reluctantly agrees to help.

Posing as a schoolgirl, enter Mindy D'Amico Headquarters and kills almost all accomplices in the loft, but left without ammunition and in the close-driven. As one of the villains D'Amico is on the verge to shoot her with a bazooka, Kick-Ass, armed with a jet-pack equipped with a Gatling guns that Big Daddy had purchased before his death, kills men remained. Red Mist and then takes another knock. Hit-Girl fights D'Amico, but is soon overwhelmed. While D'Amico was about to shoot Hit-Girl, Kick-Ass fire the bazooka, D'Amico blowing out the window, where it exploded in the air. Red Mist is to find hit and kick-ass girl with jet-pack to escape.

Hit-Girl and Kick-Ass withdraw from the crime to a more normal life. Mindy, now ex-partner custody sergeant Damon's Marcus Williams, Dave joined the school. Dave explains how a new wave of superheroes inspired by his exploits. We see Red Mist finish in the shot put a new mask, breaking the fourth wall, the audience repeats the line from the 1989 Batman movie, "wait a world of superheroes, huh? Until you experience one of mine."