One day, the eldest son, Kohei, wakes up in the morning and realizes that he had a wet dream. Mistaking ejaculation for an illness, Kohei hides it by pretending that he had wet the bed, but when his mother tells him that wetting the bed is shameful, he bursts into tears out of frustration... His older sister, Tsuna, soon learns the truth from her younger brother, but is unable to explain it verbally and decides to give him a practical sex education...