Seasoned detective Seo Do-chul (Hwang Jung-min) and his major crime investigation unit tirelessly track down criminals day and night, often at the expense of their personal lives. When a professor's murder reveals a connection to past cases, suspicion of a serial killer arises, throwing the country into turmoil. As the major crime unit deepens its investigation, the killer taunts them by publicly releasing a trailer online, pointing out the next victim and adding to the chaos. To deal with the escalating threat, the team introduces idealistic rookie officer Park Sun-woo (Jung Hae-in), leading to an unexpected turn in the trajectory of the case.