Ayumi, a university student, invites her boyfriend Kenji over to her house while her parents are away on a trip. They start having sex without being noticed by her younger brother Ma-kun, but they are found in the middle of the sex session. Ma-kun, who has no knowledge of sex, mistakenly thinks that his sister is being bullied. In order to tell her precious younger brother the truth, Ayumi takes it upon herself to give him sex education.