The narrative begins with ten people resetting one year under Dr. Lee Shin. Detective Ji Hyung-joo prevents the death of his senior colleague Park Sun-ho, and Shin Ga-hyun avoids her paralyzing hit-and-run. However, deliveryman Park Young-gil dies at the reset transition, followed by the suspicious deaths of Seo Yeon-soo (Ga-hyun's hit-and-run driver), lottery winner Ko Jae-young, and other participants.
In episodes 15–16, Lee Shin drops the bombshell that in the original timeline, every resetter invited was already dead. Lee Shin is not a deity, but a desperate mother who has repeated the same year seven times attempting to find a timeline where her terminally ill daughter survives; she used the resetters as clinical test subjects to see if death could be cheated. Meanwhile, elderly cafe owner Hwang No-sub is exposed as a wealthy sociopath faking paralysis, exploiting the loop alongside Lee Shin to prolong his life.
The ultimate twist is that the serial killer targeting the resetters is Detective Park Sun-ho, Hyung-joo's partner. Sun-ho was an undetected serial murderer whose original death at the hands of a vengeful ex-convict was the very tragedy Hyung-joo rewound time to prevent. By saving Sun-ho, Hyung-joo inadvertently set the predator free. Realizing the resetters were connected to his past cases, Sun-ho systematically slaughtered them while using his badge to conceal evidence.
In the climax, Ga-hyun sacrifices her life by taking a fatal blade meant for Hyung-joo. Devastated, Hyung-joo apprehends Sun-ho and chooses to execute a final, solitary reset. Returning to day one alone, he arrests Sun-ho for past cold cases before any murders occur. The timeline is restored to peace: Hwang No-sub's scheme is dismantled, the resetters live unaware of their averted fates, and Hyung-joo meets Ga-hyun at her book signing, where her dog Maru recognizes him and the two begin their bond anew.