Twenty years ago, her husband died in a traffic accident, leaving her to raise her son alone. Her son, now a high school graduate, refuses to work and stays shut inside the house. Wanting him to experience the outside world, Mio invites him on a hot spring trip. Burdened by past mistakes as a mother and regrets she could never atone for, she surrenders everything—her love, her guilt, her longing—into a forbidden mother-son affair.