Vampire Bokhee (uke) picks up Demon Mariam (seme) in night club, both expecting to make a meal out of their prospective partner. It goes a little differently though. Mariam accidentally drinks from a vile containing Bokhee's blood, which the vampire keeps in his pocket. Because of this mishap, Bokhee is now only able to drink from Mariam and the two are bound together in this way until either of them dies. Which will take a while considering both of them are immortal. As they figure out their situation, they find themselves in a sexual relationship that is steadily turning towards romance with both realizing they kind of actually like the other immortal supernatural being and staying together for eternity doesn't actually sound all that bad.
Their connection is tested when Bokhee is suspected of being a serial killer and the overzealous police detective on the case will stop at nothing to see the monster he's found is brought to justice.
WARNING: SPOILERS AHEAD
The second couple is so darn tragic, like, I actually feel kind of sorry for them..? Seolwoo is so mentally fucked up, any form of stability comes from his attachment to Jang-Hyeok. Jang-Hyeok himself is depended on Seolwoo because of trauma. They rely on each other for emotional stability and they both just shatter when they lose the other: Jang-Hyeok when Seolwoo is confirmed as the killer and Seolwoo when he accepts that Jang-Hyeok is dead.
At least that's what I've read into it.
I'm Yours, Blood and Soul