The King: Eternal Monarch akan mengisahkan raja di Kekaisaran Korea, Lee Kon (Lee Min Ho) yang menyukai sains. Lee Kon berkeinginan menutup pintu yang menghubungkan dunianya dengan Republik Korea, meskipun ia harus menghadapi bisikan dari iblis. Selain itu, terdapat karakter lain, yakni seorang detektif polisi kriminal Republik Korea bernama Jung Tae Eul / Luna (Kim Go Eun) yang punya prinsip selalu ingin melindungi cinta, manusia, dan kehidupan lain. Lee Kon dan Jung Tae Eul lantas bekerja sama menutup pintu yang mengubungkan dua dunia mereka.
Yi Gon is the third Korean emperor of his generation. His citizens regard him as the perfect leader. But behind this flawless appearance, hides a deep wound. When he was eight years old, his father was murdered before his eyes following a coup. Today, instead of respecting his filial duty, he prefers to escape the palace to attend university conferences. During one of his escapades, he sees himself propelled into a parallel world where he meets Jung Tae Eul, an inspector with whom he teams up with to defeat the criminals but also close the door between their two worlds.