The film is adapted from the novel of the same name by Guillaume Musso, a best-selling novelist published in 22 countries around the world. It tells the story of a doctor who returns to the past and meets himself decades ago, and travels back and forth between the past and the present to save his deceased lover. Kim Yoon-seok plays the male protagonist in the present, while Byeon Yo-han plays the male protagonist in his youth in the past.