Tokyo Metropolitan Cromartie High School is a place of legend, a dumping ground for delinquents so notorious that it has been destroyed and rebuilt six times. The film opens with a mockumentary-style recap of this chaotic history before introducing Takashi Kamiyama, a mild-mannered and academically gifted student who enrolls at Cromartie to be with his only friend from middle school. His friend, however, fails the laughably simple entrance exam, leaving Kamiyama to navigate the school alone. Due to his unflinching calm in an environment of pure mayhem, everyone immediately assumes he is the toughest student in the school.

As Kamiyama attempts to survive his first year, he befriends a small circle of bizarre classmates. These include Shinjiro Hayashida, a friendly delinquent with a fake purple mohawk; Akira Maeda, a self-proclaimed undefeated fighter who is constantly ignored; and a student named Mechazawa, who is clearly a robot despite the fact that no one, including himself, ever mentions it. The student body is rounded out by a mute, mustachioed student who looks exactly like Freddie Mercury and rides a massive horse named Black Dragon, as well as a gorilla who is smarter than most of the humans.

The narrative shifts when a rich transfer student, Takeshi Hokuto, arrives with plans to use his wealth to take over the school. However, his schemes are largely ignored by the other students. The real conflict emerges when a group of space apes, resembling villains from the 1971 tokusatsu series Spectreman, arrive on Earth with a plan to turn the students of Cromartie into mindless slaves for world domination. In response, Kamiyama organizes his friends into a makeshift Global Defense Force, leading to a climactic battle that parodies science fiction films, martial arts epics, and high school sports dramas. The film resolves not with a traditional victory, but with a return to the absurd status quo, where the school spirit is restored and the surreal daily life of Cromartie High continues. While the live-action film faithfully adapts the characters and tone of the manga and anime, it is a standalone movie released in 2005 rather than a television series.