After spending time at a juvenile detention center, teenage delinquent Saki Asamiya is given the opportunity to redeem herself and postpone her mother's potential death sentence by working as an undercover police officer. Equipped with only a special yo-yo, Saki's mission is to return to her high school to investigate corruption and mysterious student deaths. However, to uncover the truth about her school's situation, she must face the wealthy and powerful Mizuchi sisters, who moved in and seized control following her previous expulsion.