Iroha, a student who has always lived alone with her mother, suddenly gains an older stepsister, Rika, after her mother remarries. Despite their different upbringings, the two quickly grow close. Iroha learns that Rika became aware of her attraction to women during her own school years, having had a girlfriend and casual partners since then. As their bond deepens, Rika begins to develop feelings for Iroha that go beyond sisterly affection, leading to a turning point in their relationship.