In their fourth year of university, childhood friends Kousuke, Nagisa, and Ryutaro have maintained a long-standing sexual relationship. Whether it's love, friendship, or mere pleasure remains ambiguous, but the arrangement has provided them with a comfortable dynamic. Meanwhile, Nagisa's mother, Eri, a renowned fashion designer who had been living in Paris, suddenly returns to Japan and moves in with her daughter. When Kousuke and Ryutaro meet Eri for the first time, they are stunned by her youthful beauty and elegant charm. Kousuke, in particular, feels a deep emotional pull, reminded of his late mother. That night, Eri reveals she was a close friend of Kousuke's mother and had once been in love with his father. Nagisa begins to sense Kousuke's growing feelings for her mother and grows uneasy. Eri's return disrupts the trio's balance, stirring hidden emotions. Torn between her current boyfriend Tatsuro and lingering feelings for Kousuke's father, Eri's inner turmoil intensifies. Questions also arise about Nagisa's true paternity. That night, overwhelmed by conflicting emotions, Eri gives in and seduces Kousuke—setting the story on a new, irreversible path.