Koukou Debut Shita Itoko ga Mount Tottekuru.

A younger cousin, three years younger, was once an ordinary girl but has changed since starting high school, becoming more alluring. The narrator believes the cousin has begun mounting them, though confusion and emotional moments arise. The cousin's appearance has transformed, but is that all that has changed? The story explores a dynamic between a proper older cousin and a laid-back younger cousin in a one-on-one romantic comedy.