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 learns that her sister's husband cannot perform sex that a woman feels good in... let alone massage breasts. Himari, who cares for her younger sister, thinks that the relationship between the couple is in danger if things continue like this, and, worried about 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...