Kumiko “Yankumi” Yamaguchi enrolls at Shirokin High School as a math teacher. Assigned to teach class 2‑4, she finds herself in charge of a group of delinquent students who immediately test her resolve. Despite their antics, Yankumi quickly earns their respect. Unbeknownst to the school, she is the heir to the powerful Oedo Group, a yakuza clan. Balancing her duties as a teacher with the need to conceal her criminal heritage, Yankumi navigates the challenges of both worlds.