Yui Nanase is a 29-year-old married to an older man. After her husband comments that she has "gained a little weight," she decides to start dieting. As she throws herself into exercise, her stepson gradually becomes attracted to his young, alluring stepmother, indulging in uncontrollable fantasies. One day, after witnessing firsthand his stepmother's unsatisfying sex life with his father, the stepson can no longer restrain himself and takes action.