Imouto ga Brocon de Aru koto wo Ani dake wa Shitteiru.

A high school student discovers that his younger sister, Akari Namiki, is intensely obsessed with him. She is intelligent, attractive, and stylish, while he sees himself as average. In his third year of middle school, he finds a notebook in her room that meticulously documents his actions and reveals her deep affection for him. He also uncovers hidden cameras and listening devices in his room. Later, in his second year of high school, he learns of her plan to isolate him and ensure his dependence on her. Determined to prevent things from escalating, he resolves to maintain only a brotherly relationship with her.