A police officer A (Gyeon Joon) lives alone. He had a fianceé but she died 1 month (or even a week - don't remember) prior the wedding. He's depressed even after 2 year (I think). He's investigating an occullt group. Some sh*ts happen and now he has a demon Cain (nickname) stuck to him cause he's the only "door" that doesn't break after it is used by a demon (basically the door can be oppened by a climax - this can be done by magic or by pleasure aka sex. The door is like a gate to another dimensions - Hell, Heaven, Earth). So now Cain sticks with him while P.o. A tries to live the most normal life possible (doing his job and survive) - but he has to deal with lots of BS thanks to Cain. Anyway Cain is being hated by Satan and his demons so he gets into a fight pretty often + he has a friend demon Sid that betrayes him.
+ There is an police officer B (Jin Yeong) who is friend of P.o. A and he has a crush on him. And Sid uses him (ch. 11 Sid gets fucked while P.o. B is under Sid's spell - he sees Sid as P.o. A). During all this BS the demons are searching for a doctor (that's from another dimension and can make clones of himself). Anyway after more BS Cain falls for the P. o. A but doesn't know it is love so he tried to "get rid of this weird spell" by disciplning P. o. A (ch. 17 - 18). In ch. 19 Cain goes on a rampage cause P. o. B saw these twon outside and P. o. A tried to get away from Cain but Cain didn't allow it. In the end the 2 P. o. went their way and Cain was so jealous - that's why he went on a rampage.
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Dear Door