Ryo moves into the home of his sister Harumi and her family, where he is welcomed by the kind mother, Kasumi. While Harumi shows an interest in him, an accidental encounter in the shower sparks a hidden passion between Ryo and the neglected Kasumi. Despite the guilt of betraying her husband, Kasumi gives in to her desires, leading to a secret and illicit affair within the household.
During a beach trip, Kasumi struggles with the guilt of her affair with Ryo and the discovery of her husband's infidelity. Believing she must prioritize her daughter Mio, Kasumi attempts to end her relationship with Ryo to fulfill her duties as a mother. However, after a period of separation and reflection, she realizes her love for Ryo is undeniable. The two eventually reunite and confess their deep feelings, choosing to commit to one another despite the turmoil of Kasumi's marriage.