In a wealthy and happy household, forbidden relationships beyond familial bonds are about to break free from their restraints. Kameinosuke, the third generation head of the Goto family, remarries and welcomes a new wife, Suzu. On the eve of his birthday, the family's harmony begins to shift. Kameinosuke has one child, while Suzu has two step-sisters from a previous marriage. A family photo session is planned for the birthday celebration, but the next day, once the photos are done, the parents go out for dinner. Seizing the opportunity, older sister Reira appears before her stepbrother Manabu...