Sarara was entrusted by her strict father with educating her younger brother Akira. However, Akira's nonsensical behavior always made her father angry, and Sarara was stressed out. One day, while taking a bath with Akira, Sarara was fixated on his penis. Sarara, who was aroused by her own brother, decided to teach him "sex education" even though her father had not entrusted her with it.