Yurika, pressured by her estranged husband to divorce, finds herself nearing her emotional breaking point. Despite this, she continues living day to day with her son, Kōichi, gradually losing touch with her true self. Once warm and gentle, she has grown colder under the weight of daily stress and loneliness, now often screaming hysterically at night. Witnessing this repeatedly, Kōichi begins to lose his vitality. One day, he confides in his friend Satoshi, only to uncover a shocking truth—Satoshi has been engaged in an incestuous relationship with his own mother, Sumika, for years. Leaning in close, Satoshi whispers to Kōichi: "Let’s swap mothers and have some fun." Those words become the catalyst that drastically alters their lives forever.