Four mothers-in-law enter secret relationships with their adult sons-in-law despite knowing they must never be discovered. Tsubaki listens to her son-in-law complain over drinks, then gradually accepts his embrace out of loneliness. Marika knows her daughter's marriage is strained and lets her son-in-law into her bedroom. Natsuko yields when he touches her body and seeks forbidden pleasure, hoping it will ease her loneliness. Aya, whose married sex life never went beyond the missionary position, is shocked and aroused by what she learns of her daughter's sex life.