Neuro Nougami is a demon from hell, who eats riddles. Having eaten all the simple riddles in hell, Neuro is now ready to go after more complicated and tasty riddles from the human world.

However, Neuro can only eat the riddles when they are solved and the evil energy is released from the mystery it belongs to.  To hide his identity as a demon, Neuro makes Yako act as his partner "famous student detective from the Net", against her will.  As a team of detectives we have never seen, the two tackle and solve riddles after riddles from the human world to satisfy the demon's appetite.