The story revolves around , the stoic and handsome Emperor of the Kingdom of Corea—a constitutional monarchy in an alternate reality. On the other side of a mystical portal lies the Republic of Korea (modern-day Seoul), where Jung Tae-eul (Kim Go-eun) works as a fierce police detective.

The drama follows Lee Gon, the stoic yet romantic Emperor of the Kingdom of Corea, who finds a mystical door to a parallel world—modern-day Republic of Korea. He crosses paths with Jeong Tae-eul, a fierce detective, leading to a love story that defies time, space, and fate.

The King