Hyeong Ju is a police detective who tackles his demanding work with unwavering loyalty to his teammates. However, when one of his colleagues passes away, Hyeong Ju loses his sense of purpose and spirals into despair. Ga Hyeon, a famous webtoon artist, is the creator of a thriller webtoon titled "The Hidden Killer." Her life revolves around investigating cases 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 when Hyeong Ju and Ga Hyeon are at their lowest, they both receive a mysterious call. The caller offers them the chance to relive the past year. What will they choose?