Rumors of a serial killer stalking the city reach four magical girls, Arisa Narumi, Chisato Shion, Haruka Kanade, and Matsuri Hinata, who suspect a witch might be behind the murders and go out to patrol. Instead they find themselves hunted by the killer, a magical girl named Suzune Amano who has decided that killing other magical girls before they can turn into witches is a mercy. The four must figure out how to stop someone whose twisted logic makes her nearly impossible to reason with.