One day, when Kohei, the eldest son, woke up in the morning, he noticed that he had a wet dream. Kohei misunderstood ejaculation as a disease and hid it by disguising it as oneesho, but his mother told him that oneesho was shameful and he cried because he felt helpless. Immediately after hearing the truth from her younger brother, her older sister Tsuna couldn't explain it clearly, so she decided to put sex education into practice.