"If you let us stay over... we'll do something naughty with you." Mitsuhiko Okaya, an unremarkable man, was betrayed by both his wife and a junior colleague. Mocked at work as a useless, sidelined editor, Mitsuhiko falls into despair — until two twin high school girls appear before him. Strangely, although they seem unfamiliar, he feels a sense of déjà vu. One is Akari, the lively, cheerful younger sister who loves teasing him; The other is Nanako, the shy, clumsy, yet secretly lewd older sister. It turns out they were once his students back when he worked part-time at a cram school during his own school days...