Rimu and her Yuu, who will be graduating next year, have kissed but have not gone any further. They have promised to wait until they are adults before having a physical relationship. Rimu comes to her boyfriend's house to play and meets Yuu's father and Otoru, who treat her like a daughter. That day, the train stops and Rimu is unable to go home, so she has to stay the night. The next day, while Yuu is out, Otoru gives Rimu an aphrodisiac.