During a family getaway to a hot spring, a sudden bout of memory loss leaves Airi in a childlike state, leading her to call her sister Marina "Mama." Taking advantage of the confusion, the father indulges in forbidden encounters with Marina, unaware that Airi's memories have already returned. After a brief moment of emotional turmoil and a near-departure, the truth comes to light, and the family returns home with their twisted bond stronger than ever.