Kouki is under pressure to choose one of his sisters as his fiancée before summer vacation ends. While struggling with the decision, he shares an intimate encounter with Saori, the household's shut-in maid, after she follows him into the rain. This leads to a confrontation with his eldest sister, Yurina, who decides to claim her own time with him while reminding him that he must eventually make a final choice.
Kouki is tasked by the Hanabishi family patriarch to spend the summer living with the four daughters and choose one of them as his bride to fulfill a long-standing promise between fathers. As he navigates his complex emotional and physical bonds with Yurina, Momone, Saori, and Anzu, Kouki struggles with the pressure to pick a single partner. Ultimately, realizing he loves all four sisters equally, he declares his intention to marry every one of them, even if it means changing the law.