After the death of his beloved girlfriend, police officer Kyungjoon just wants to be left alone to water his house plants and catch criminals. But after a police investigation into a mysterious cult goes supernatural, Kyungjoon finds himself unwillingly caught up in a world of demons, succubi and monsters - and the worst of them all is a demon king calling himself "Lord Cain." It turns out that in order to survive, Cain needs mana that can only be accessed through a "door" inside Kyungjoon. The problem is... how is he planning to open that door?