After being mugged and witnessing a brutal attack on a fellow classmate, Dave decides to take matters into his own hands and becomes a vigilante, donning a green and yellow costume and calling himself Kick-Ass. At first, his attempts at fighting crime end in disaster, but a YouTube video of him taking on some thugs goes viral, making him an overnight sensation.
Meanwhile, a father-daughter duo of vigilantes, Big Daddy and Hit-Girl, are on a mission to take down a powerful mob boss, Frank D'Amico. When Kick-Ass unwittingly crosses paths with them, he becomes caught up in their mission and helps them take on Frank and his henchmen. Along the way, he also falls for a girl named Katie, but struggles to balance his secret identity with his normal teenage life.
The film is known for its graphic violence and controversial portrayal of a young girl, Hit-Girl, who is trained to be a ruthless killer by her father. Despite the controversy, the film was a critical and financial success, and spawned a sequel, Kick-Ass 2.