One day, after marrying and moving into her husband's family home, she began living with his younger sister, schoolgirl Mitsuki. Their life together started peacefully, but one morning, a sudden kiss left her confused. At first, she thought it was just a prank, but later discovered that before the marriage, her husband had mentioned, "It'd be nice if my older sister were around the house." From that moment, she began sensing something different in Mitsuki's feelings, and her own emotions started to waver and grow.