Ryouta Murakami has spent ten years carrying the guilt of a childhood friend he calls Kuroneko, who died trying to prove to him that aliens exist. To keep the promise he made her, he joins his school's astronomy club. When a transfer student named Kuroha Neko arrives bearing an uncanny resemblance to Kuroneko, she reveals she is a 'witch' with supernatural powers, escaped from a secret research facility. The witches depend on special medication to stay alive, and Ryouta ends up hiding Neko and the girls who fled the lab with her.