Takuya, a college student, finds himself unexpectedly attracted to his older stepsister, Aika, a former delinquent and nursing mother who has returned to the family home. Aika's provocative behavior and lack of boundaries lead the two into a risky, clandestine sexual affair. Despite her guilt toward her husband and children, Aika becomes addicted to Takuya's advances, leading to a dangerous game of secrecy and forbidden passion within their own household.
Taku and his family take a trip to Atami, where he and his married stepsister, Aika, indulge in a series of risky sexual encounters at the beach, their inn, and a mixed-gender hot spring. Following the trip, Aika becomes pregnant with a third child, further deepening their forbidden secret affair while she remains married to her husband.