William Twining is a rich 19th-century British aristocrat - and a well educated genius - who only believes in hard scientific facts. He doesn't believe in gods or any other superstitious fairy-tale nonsense. He is in all ways a "realist". However, he soon finds out that these things he mocks are, in fact, very real.
When William's school fees aren't paid, he returns home to find out that his uncle has lost the family fortune and the only thing left is an empty mansion. Convinced that there must be something of value left, he searches the cellar, where he manages to accidentally summon a demon called Dantalion.
Refusing to believe what is before him, he has the human-looking Dantalion arrested. Continuing his search, he passes through the summoning portal and arrives at the demon's abode in the underworld.
Will William be converted - and will he be able to escape from hell? Check out the show on Crunchyroll to find out.