Title: The King: Eternal Monarch
Plot: Lee Gon, a modern-day Emperor of the Kingdom of Corea, attempts to cross the barrier into an alternate reality where the Republic of Korea exists in the Kingdom's stead. He comes across detective Jung Tae Eul, whom he recognizes from an identity card he obtained during the turning point of his childhood, his father's assassination. Lee Gon's half uncle, Lee Lim, who assassinated the previous king, Lee Ho (Lee Gon's father), is in hiding and assembling armies whilst traversing back and forth between the two parallel worlds.
Writer: Kim Eun Sook
Director: Baek Sang Hoon
This is the first kdrama couple that I actively ship in real life. The chemistry!!
I really enjoyed having parallel universes in this drama. Staring at Lee Min Ho all episode long didn't hurt either.