Naoki has been married for three years with Karen, a nurse. He lives a happy life with her, but he sometimes thinks, "Do I really love my wife?" One day, Akira, his senior, invites him to his house and Naoki accidentally sees Akira and his wife, Mayumi, having sex there.... He can't forget the scene.... Several days later, Naoki receives a call from Mayumi. "Did you see us having sex the other day...? Can you sleep with me just once? It'll make it clear how much I love my husband." Naoki is surprised to hear that, but they finally sleep together.... Once, twice, thrice.... They are gradually addicted to their secret affair....