In a world where some humans are known as “doors”, who have a demonic symbol tattooed somewhere on their body.
Humans are randomly chosen, while some are in cults simply to be chosen as doors. And one day when the demon kind his Majesty ‘Cain’ comes over to Earth—with his right hand man and servant Sid,—he comes in contact (accidentally) with a cop named Kyungjoon whom he marks as his door.

Throughout the story, Cain and Kyungjoon slowly grow feelings for each other, while uncovering backstabbing friends, and stopping the world from falling into chaos.