Himari, who loves her younger sister, stays at her sister's house during a sightseeing trip and plots to further enhance the couple's relationship. However, Himari learns that her brother-in-law cannot make women orgasm, let alone skillfully grope breasts. Concerned for her sister's marriage, Himari takes it upon herself to teach her brother-in-law how to properly grope breasts, luring him with large breasts and secretly practicing penetration under the guise of a 'how-to sex' guide.
