Himari, who cares for her younger sister, plans to stay over at her sister's house while sightseeing and to further improve the relationship between the couple. However, Himari hears that her sister's husband cannot perform sex that women can enjoy... and is even terrible at massaging breasts. Himari, who cares for her younger sister, thinks that the relationship between the couple is in danger if things continue like this, and out of concern for the couple's relationship, she meddles and seduces her brother-in-law with her big breasts to teach him how to massage breasts, and secretly practices sex with him under the guise of a "How to Sex" class...