After becoming homeless, college student Kosuke is taken in by the stunning Aoi Sakura to live with her and her sister, Madoka. While the arrangement is intended to lead to a marriage between Kosuke and Madoka, Aoi takes it upon herself to "train" Kosuke sexually, claiming she must exhaust his lust to ensure his future faithfulness. When Madoka discovers their illicit encounters, she is forced to confront the provocative and unconventional nature of their new household dynamic.
Kosuke becomes entangled in a complex love triangle when Madoka attempts to steal him away by claiming Sakura is only using him as a replacement for her deceased husband. However, Sakura eventually confronts her past at her husband's grave, proving that her love for Kosuke is genuine and independent of her grief. With Madoka's eventual blessing, Sakura and Kosuke cast aside their doubts and commit to a new future together.