Dave Lizewski, also known as Kick Ass, narrates his extraordinary life story, illustrating how average individuals can transform into superheroes.
Six months prior, Dave is just a typical geek, surrounded by his close friends, Toddy and Marty, and spending much of his time at a comic book store. Following the unexpected death of his mother due to an aneurysm when he was 14, he struggles to find connection in his life. His affection for Katie Deauxma is unreciprocated, as she hardly acknowledges him. A genuinely kind-hearted guy, Dave tries to form friendships, including one with Chris D'Amico—the lonely son of a local mobster—who is kept isolated by his overbearing father. One fateful day, after discussing superheroes at the comic café, Dave and Toddy become victims of a mugging by gang members. This event ignites a spark in Dave to don a superhero persona and combat crime. He orders a vibrant green diving suit online and commits himself to intense training, gearing up to be a true crime fighter.
Weeks later, after his exhaustive training, Dave stumbles upon the two robbers attempting another crime and decides to put his skills to the test. However, things take a tragic turn when he is brutally stabbed in a confrontation. As he clings to life in the ambulance, he desperately pleads with the paramedic to conceal his superhero suit. Dave is later admitted to the hospital in a humiliating state (which is heavily insinuated to be due to gay prostitution).
Months roll by, and Dave returns to school now equipped with metal plates and heightened pain tolerance. To his astonishment, he discovers that Katie is suddenly interested in him. Marty fills him in that Katie has a penchant for "losers," and the school rumor mill has turned Dave into the talk of the town, incorrectly branding him a gay prostitute. Nevertheless, Dave's resolve to be a superhero remains unwavering. He sets up a MySpace account to rally support and take on cases. One night, he intervenes when a group of attackers beats a defenseless man outside a diner, earning him notoriety as Kick Ass on the internet.
As Dave rises in fame, an underworld battle brews across town. Frank D'Amico, a powerful mob boss, learns of a mysterious vigilante resembling Batman who has disrupted his drug operations. In a fit of rage, he tortures one of his associates for information, and when the news of Kick Ass reaches him, he decides to eliminate him. Meanwhile, Damon MacReady, a disgraced ex-cop, is training his daughter, Mindy, to seek revenge against D'Amico's crew. They observe Kick Ass's exploits; while Damon remains unimpressed, Mindy is inspired.
Damon, consumed by vengeance for his murdered wife during his prison sentence, sets his sights on annihilating D'Amico's organization. Despite warnings from his friend Marcus, Damon becomes increasingly unstable in his quest. As Kick Ass, Dave continues to save people while simultaneously growing closer to Katie, who reveals that a dangerous meth addict named Rasul has assaulted her. Driven by a mix of fear and valor, Kick Ass attempts to confront Rasul, but his mission turns desperate. Just as he faces peril, Mindy—disguised as Hit Girl—heroically dispatches the thugs with lethal efficiency, rescuing Dave from certain doom.
Mindy introduces herself as Hit Girl and informs Dave of her willingness to partner up. However, feeling inadequate, Dave is disheartened and has a sobering moment of self-doubt when they visit him later that night. Encouraged by Big Daddy, Damon, and Hit Girl, they advise Dave to reach out when in need.
As tensions rise, Frank, feeling threatened by the presence of vigilantes, kills a Kick Ass impersonator, incorrectly believing he has disposed of the true hero. Chris D'Amico, the actual son of Frank, proposes to step into the superhero world as Red Mist, aiming to entrap Kick Ass. Chris lures Dave into a trap using shared adventures and fake camaraderie, but it leads them to a horrific discovery of death and despair.
Kick Ass finally decides to unveil his true identity to Katie, which results in a disastrous misunderstanding. Still, their relationship blossoms amidst the chaos. As Frank continues his manipulative quest, he uncovers evidence of Big Daddy's ferocity, devising a plan for revenge.
In a harrowing showdown at D'Amico's headquarters, events spiral further as Mindy and Dave confront their enemies head-on. During a fierce battle, a heavily wounded Big Daddy sacrifices himself to save Hit Girl and Kick Ass. The two heroes unite, ready to avenge the loss of Damon and restore their city.
Hit Girl shows her unmatched combat prowess, infiltrating D'Amico's fortress while Kick Ass devises a plan of airborne assault much to the shock of onlookers. The climactic fight leads to chaos and betrayal. Chris's transformation into a villainous Red Mist highlights the cyclical nature of their conflict.
In the aftermath, while Marcus takes custody of Mindy, the duo of Kick Ass and Hit Girl contemplates their future. A voice-over reveals Dave's retirement from crime-fighting, only for a sudden twist as Chris—adopted a new identity—vows revenge against the new generation of superheroes.
Ultimately, Kick Ass becomes a cultural icon, symbolizing the rise of heroism amid ordinary lives while leaving viewers with a thrill of anticipation for what's to come in a world of newfound vigilantes.
