On the eve of her son "Yuuki's" exam, mother "Reiko" notices he's acting strangely and grows concerned. One day, when Reiko returns home, she sees Yuuki emerging unnaturally from the bedroom. Inside, she finds her own underwear stained with his semen, leaving her shocked and confused. Gently guiding Yuuki, Reiko decides to teach him sex education using her own body. Through this process, their relationship gradually transforms into something entirely new.