"Mystery" refers to enigmas, which release vast amounts of energy when solved—enigmas filled with malice (ranging from pranks to deep-seated hatred), such as murder cases. The protagonist, Neuro Nogami—a mutant creature from the demon world—is a unique being that feeds on "mysteries." After consuming all the "mysteries" in the demon world, he arrives in the human world in search of the ultimate "mystery." Upon his arrival, he encounters the heroine, Yako, who is devastated after her father is murdered. Seizing the opportunity, Neuro reveals his true form and forces Yako to become his agent. Through Yako, he successfully solves several cases. Thanks to Neuro's keen intuition for "mysteries," Yako becomes a famous detective, and the two establish a detective agency together, jointly pursuing the ultimate "mystery."