After divorcing her husband, Reiko lives alone with her son, Hiroshi. The two have a habit of bathing together, but one day Hiroshi says to her, "My penis gets hard when I bathe with you, Mom," leaving Reiko shocked and flustered. A few days later, Hiroshi stays overnight at a friend's house and is shown adult videos, learning about sex for the first time. That night, unable to sleep, he discovers Reiko bathing alone and, torn between awkwardness and curiosity, slowly approaches her.