Dave Lizewski, portrayed by Aaron Johnson, finds himself in a state of ennui after stepping back from his role as the vigilante Kick-Ass. Seeking to rekindle his heroics, he begins training with the formidable Mindy Macready, played by Chloe Moretz. However, Mindy is struggling with the aftermath of her father Big Daddy's tragic death and is battling severe isolation. In a bid for companionship and purpose, she agrees to team up with Dave.
Meanwhile, Chris D'Amico, whose life has been turned upside down, gets into a heated argument with his mother over the loss of his father. Frustrated and despondent, he accidentally causes a tanning bed to collapse on her, resulting in her untimely demise. Now in command of his family’s wealth, Chris transforms himself into the villainous Motherfr, played by Christopher Mintz-Plasse, vowing to exact revenge on Kick-Ass.
As Mindy’s guardian, Marcus, discovers her continued crime-fighting endeavors, he implores her to relinquish her superhero aspirations. In the midst of all this, Dave’s girlfriend, Katie Deauxma, grows suspicious of his odd behavior and ends their relationship under the misconception that he is cheating on her with Mindy. Embracing his vigilante side, Kick-Ass aligns with the superhero group Justice Forever, led by Colonel Stars and Stripes. This unconventional team includes quirky members like Battle Guy, played by Dave's friend Marty, Dr. Gravity, Insect Man, Night-Bitch, and Tommy's parents, who are on a quest to find their missing child.
The group initially dedicates themselves to benevolent acts like volunteering at soup kitchens, but soon they tackle larger threats, such as shutting down a sketchy brothel. During this time, Kick-Ass develops a romantic connection with Night Bitch, while they also attempt to recruit their friend Todd into the superhero fold. Unfortunately, their efforts result in Todd feeling alienated, as they deride his chosen name, Ass Kicker, as a poor imitation of Kick-Ass.
Mindy attempts to live a seemingly normal life but struggles with bullying from popular girls, particularly Brooke. After impressing everyone with her dance audition, she faces jealousy and retaliation. Chronicling the ups and downs of her life, Dave continues to encourage Mindy to reclaim her assertiveness, which leads her to retaliate against the mean girls with clever insults and shock tactics, ultimately resulting in her suspension from school. Complications arise when Dave's father discovers his son's secret identity after stumbling upon his superhero costume.
As tensions build, the Motherf****r constructs a menacing gang of villains, which leads to destructive encounters, including the brutal murder of Colonel Stars and Stripes. His henchwoman, the intimidating Mother Russia, becomes a formidable foe, slaughtering police officers and inciting a crackdown on vigilantes. An IP trace leads police to wrongly accuse Dave's father of being Kick-Ass, prompting Mr. Lizewski to take the fall to protect his son.
Upon learning the truth about Dave’s situation, the Motherf*r orchestrates a chilling fate for Mr. Lizewski, sending a photograph of his death to Dave, revealing his own identity as Chris D'Amico. Heartbroken and vowing not to don his Kick-Ass suit again, Dave finds himself kidnapped by the Motherf*r's gang after a funeral ambush.
But Mindy is not one to let her friends down. She pursues their van and engages in a fierce battle, successfully taking down most of its occupants to rescue Dave. She persuades him to confront the Motherf****r head-on. In an explosive showdown at the villains' lair, where Kick-Ass is intended to be fed to a shark, Mindy and Dave's friends unite against their adversaries. Throughout the chaos, Mindy displays her resourcefulness, ultimately defeating Mother Russia after cleverly using adrenaline for an intense boost of power.
After an iconic struggle on a rooftop, the Motherf****r succumbs to his end when he refuses Dave’s help, plunging into his own shark tank where he meets a gruesome fate. Following these harrowing events, Justice Forever decides to retire from their costumed exploits.
As the story wraps up, Mindy takes Dave home on her motorcycle. Aware of the risks she faces, she decides to leave New York but not before declaring that he is now the hero the city needs. With a final kiss and a heavy heart, she departs, leaving Dave to embrace the mantle of a true hero and gear up for future challenges.
In a humorous post-credits scene, the Motherf****r unsuccessfully pleads for water from a hospital bed, lamenting about the grim consequences of his descent into villainy.
