あの鬼教師が僕の姉になるんですか？

父親が娘のいる女性と再婚したため、少年は自分に姉ができたという知らせに最初は大喜びする。A boy is initially overjoyed at the news of him having an older sister, due to his father remarrying to a woman who has a daughter. That is, until he finds out that his new sister is actually his school's strict PE teacher, Shina. With his father going off to live with his new wife, the boy will live with alone with his new sister. In their new life together, the boy finds out that Shina at home is completely different from Shina at work: clumsy, scared of insects, and kind.