Seo-young (Park Shin Hye) has returned home after a long time. At home, she connects an old phone and a call from an unknown girl named Young Sook. Seo Yong realizes that Young Suk has been living in the same house 20 years ago and they have been building friendships ever since. One day, they make a small choice to change each other’s lives in their present. Young-sook saves Seo-Yeon’s father, who died 20 years ago, and Seo-Yeon, in turn, informs Young-Sook about the future. However, Young-Sook finds out about his terrible future, and begins to threaten her, due to an unexpected escape.