The series follows Neuro Nougami, a demon who feeds on mysteries and puzzles. He enters the human world to feast on the riddles left by criminals who generate negative energy through their planned crimes. Because demons are prohibited from being discovered in the human world, Neuro must keep his presence hidden. To satisfy his appetite, he forms a permanent partnership with Yako Katsuragi, who has her own mystery to solve. Together they investigate and solve crimes, providing Neuro with the cases he craves.