Saki Asamiya is an unlikely heroine - while serving time in prison on trumped-up charges, she's forced to become an undercover police agent to keep her mother, who's serving time on death row, from being executed. She returns to her old high school and acquires two friends - gentle artist Junko and adoring Sampei Nowake. The three Mizuchi sisters, the daughters of a corrupt politician, are using the school to launch their own schemes that include extortion, assault, drug-dealing, kidnapping, rape, and murder. In addition to her skill as martial artist, Saki fights with a police-issue yoyo, which becomes a deadly weapon in her hands.