Detective Seo Do-cheol (HWANG Jung-min) and his Major Crime Investigation Division tirelessly track down criminals day and night, often at the expense of their personal lives. When the murder of a professor reveals links to past cases, suspicions of a serial killer arise, plunging the country into turmoil. As Major Crimes delves into the investigation, the killer taunts them by publicly releasing a teaser online, indicating the next victim, and intensifying the chaos. To tackle the escalating threat, the team brings in idealistic rookie officer Park Sun-woo (JUNG Hae-in), leading to unexpected twists in the case’s trajectory.