The series focuses on Neuro Nougami, a demon who feeds on mysteries and puzzles, who comes to the human world to feast on the mysteries offered by humans who release negative energy from their planned crimes. As a demon, he must not make his presence noticed by the humans as it is the law in the human world. Because of this, he makes a permanent deal with Yako Katsuragi (who has her own personal mystery she has to solve), and together they solve crimes for Neuro's appetite.