The central plot revolves around two intertwined tragic histories from the Goryeo Dynasty. Kim Shin, a revered general, is executed by the young, insecure King Wang Yeo, who was manipulated by his evil eunuch advisor, Park Joong-heon. The King also orders the death of his wife, the Queen, who is Kim Shin's younger sister, Kim Sun. Consumed by guilt, Wang Yeo takes his own life. This series of events leads to their reincarnated fates: Kim Shin becomes the immortal Goblin, Wang Yeo becomes the amnesiac Grim Reaper as punishment, and Kim Sun is reborn as Sunny. Ji Eun-tak is a "miscellaneous omitted soul"; she was supposed to die in an accident with her mother but was saved by a merciful whim of the Goblin, which is why she can see ghosts and is destined to be his bride.
A major twist is the gradual revelation of the Grim Reaper's and Sunny's true identities, not only to the audience but to themselves and to Kim Shin. The moment Kim Shin realizes his new friend and roommate is the reincarnation of the king who destroyed his life is a pivotal climax. Another significant twist is the role of Yoo Deok-hwa. For much of the series, he is comic relief, but in a crucial moment, he is revealed to be a temporary vessel for the Almighty, a deity who directly questions the main characters about their choices.
The finale sees Kim Shin sacrifice himself by having Eun-tak pull the sword, which he uses to vanquish the ghost of Park Joong-heon. He does not find peace but is instead trapped in a snowy, desolate limbo for nine years until Eun-tak's prayer summons him back to Earth. However, the world has forgotten him, including Eun-tak, who only feels a deep, unexplainable sadness. She eventually regains her memory through a maple leaf she saved from their trip to Canada. They share a few years of happiness before Eun-tak, now 29, dies tragically in a car accident she chose to cause to save a bus of children. Her death was an act of human sacrifice, not fate. She meets the Grim Reaper, who guides her to the afterlife, promising to be reincarnated. The Reaper and Sunny, having finally reconciled with their past, also walk through the portal to their next life together. The series ends with Kim Shin, eternally waiting, as a reincarnated Eun-tak finds him once again in Quebec, fulfilling her promise to find him in every lifetime.