Sixteen years before the story begins, a massive alien spaceship crashed into Earth, leveling Graviton City and leaving behind a enormous crater. The city has since been rebuilt into the most advanced metropolis on the planet, constructed directly within the ruins of the ancient extraterrestrial vessel. The narrative follows Eiko Magami, known as A-Ko, a red-haired high school girl who possesses superhuman strength and speed, abilities inherited from her genetically enhanced parents. Despite her incredible power, A-Ko simply wants to live a normal teenage life, though her primary daily concern is more mundane: waking up on time and walking to school with her bubbly, naive best friend, Shiiko Kotobuki, or C-Ko.

This peaceful routine is shattered by Biko Daitokuji, or B-Ko, the brilliant but spoiled daughter of a wealthy and powerful local family. B-Ko develops an obsessive infatuation with C-Ko and becomes intensely jealous of A-Ko's close friendship with her. Determined to drive A-Ko away and claim C-Ko for herself, B-Ko launches a daily campaign of attacks against her red-haired rival. Using her genius intellect, she builds an escalating series of powerful mecha suits, most notably the Akagiyama 23, and leads her team of female followers in battles against A-Ko right at the school gates each morning. What begins as a personal rivalry over friendship quickly escalates into spectacular, city-shaking confrontations that parody common anime and mecha tropes.

The conflict takes a dramatic turn when a new alien spaceship arrives in orbit above Graviton City. A mysterious trench-coated agent known only as D has been monitoring A-Ko and C-Ko, reporting back to the ship's captain, the alcoholic Napolipolita. The aliens, from the Lepton Kingdom of Alpha Cygni, have concluded that their long-lost princess is hiding on Earth. In the midst of A-Ko and B-Ko's latest destructive fight, the aliens launch an all-out attack on the city, easily overwhelming the military, and successfully abduct C-Ko, revealing her to be the lost princess they have been seeking.

Witnessing their friend taken, the bitter rivals A-Ko and B-Ko agree to a temporary truce. They infiltrate the alien mothership, with A-Ko confronting the captain and agent D while B-Ko works to free C-Ko. However, the truce is fragile; once C-Ko is rescued, B-Ko immediately turns on A-Ko again, opening fire inside the ship's bridge and destroying its navigation systems. The damaged vessel plummets back to Earth, coming to rest precariously balanced atop the Military Command Tower, which is itself the buried remnant of the original spaceship from sixteen years ago. The next morning, the still-sore A-Ko and the oblivious C-Ko walk to school in their new uniforms, passing by a disheveled D and Captain Napolipolita begging for spare change to repair their wrecked ship, as a smiling B-Ko waits on the horizon, ready to begin the fight all over again.