Niina raised her son alone. Although she encouraged him to get a job and live independently, he refused to leave home. Wanting to do something special for him, she occasionally invites him on a hot spring trip. During their trip, her son suddenly confesses—he has no girlfriend and has always been in love with her. Overwhelmed by his long-hidden feelings, Niina gradually begins to take him seriously. What started as a mother's affectionate desire to protect her son's virginity becomes something deeper. Though they promised it would happen only once, their bond grows stronger, blossoming into a forbidden mother-son relationship. As their emotions intertwine, Niina surrenders herself to intense pleasure. Repeatedly entwined in passion, their taboo love story unfolds deeper with each encounter.