One morning, the eldest son, Kohei, wakes up to discover he has had a nocturnal emission. Believing it to be an illness, he tries to hide it by pretending he wet the bed. But when his mother scolds him, saying, "Bedwetting is shameful," he can't hold back his tears, overwhelmed by embarrassment and frustration. Meanwhile, after hearing the truth from her younger brother, older sister Tsuna finds it difficult to explain things with words alone. Feeling that understanding through experience is more effective, she decides to provide him with real sex education. This intimate incident within the family becomes a pivotal step in his growth—and an unexpected moment that deepens their familial bond.