Hyeong Ju is a police detective who tackles the gruelling work with unwavering loyalty to his teammates. However, when one of his colleagues passes away, Hyeong Ju loses his motivation in life and spirals into despair. Ga Hyeon, a famous webtoon artist, is the creator of a thriller webtoon entitled "The Hidden Killer". Her life revolves around investigations and turning those stories into art, but she has her boyfriend and best friend by her side. Then, a sudden accident plunges her into a dark period. Just as Hyeong Ju and Ga Hyeon are at their lowest ebb, they both receive a mysterious phone call. The caller offers them the chance to relive the past year. What will they choose?