For years since her husband took a mistress and stopped coming home, Yukari has poured all her affection onto her son Shota. But Shota suffers in silence, tormented by inner confusion over his mother's behavior—especially because he's sometimes used as her nighttime lover. One day, Shota confides in his best friend, Kohei. To his shock, Kohei reveals he's also engaged in a similar relationship with his own mother, Sawako. Then Kohei whispers a proposition: "Let's swap mothers and have some fun." With those words, the fate of both boys takes a shocking new turn.