Aika is a mother living with her husband and teenage son, Junya. One day, Junya breaks his arm in a fight with a friend, and Aika begins to look after her son, who now needs her help. However, Aika is constantly aware of her son's body as he grows into an adult, and she ends up giving him a lesson in the innocent penis. Then Junya can no longer suppress his interest in sex... Aika feels responsible, and as a mother, she uses her own body to teach him the Iroha of sex...