The story, without the spoilers.
In one world, Lee Gon is the king of the Kingdom of Corea, a modern monarchy that exists in a parallel universe. In the other, Jung Tae-eul is a detective in the Republic of Korea, practical, stubborn and unimpressed by titles. A magical flute, a traitor from the king's past, and a doorway between worlds bring them together, and their romance must survive assassins, doubles and the simple problem of living in two different countries that share the same streets. The King: Eternal Monarch is lavish to look at, with beautiful costumes, horses, palaces and two charismatic leads. But the parallel-world rules are confusing, the middle episodes slow down, and the plot asks you to accept a lot without explaining it clearly. If you love Lee Min-ho and Kim Go-eun, and you enjoy grand romantic fantasy, there is plenty to enjoy. If you want a tight, clear story, the spectacle may not be enough to carry you through.
A little curiosity. A new favourite.
Start with the story, meet the cast, and make room for one more episode.