Mark inherits an old mansion in korea from a distant relative he didn't even know existed. Once his parents convince him and his brother to actually move in he realizes that he's not alone. The house has a portal to a parallel universe in the form of a mirror in the main hall. Youngjae was the previous owner, who got through the portal by accident in the 50s with Yugyeom. When Jaebum discovers him they converse through the mirror and Jaebum decides to help rescue the trapped humans. Mark can talk with Yugyeom during the day.But Youngjae can only be seen at night.