Neuro Nougami is a demon who feeds on mysteries, and after devouring every puzzle in the demon world he crosses into the human one in search of fresh ones. He fastens onto Yako Katsuragi, a sixteen-year-old whose father's murder was ruled a suicide, and sets up a detective agency in her name, casting himself as her assistant while she takes the public credit. Using his 777 Tools of Hell, Neuro chases the killings he finds most interesting and drags a reluctant Yako along to front each case.