Detective Seo Do-cheol, Hwang Jung-min, leads the Violent Crimes Investigation Unit as they raid an illegal gambling den. Soon after, a viral video surfaces showing a masked killer executing a disgraced art professor who raped a female student, driving her to take her own life. The police realize this is part of a disturbing pattern: each victim had once perpetrated a crime themselves, and they were killed in the same way their victim had suffered. As more killings unfold, authorities suspect a serial killer is targeting those who escaped justice.
A popular YouTuber named Editor Justice covers the murders, dubbing the killer "Haechi" and presenting them as a vigilante exposing flaws in the legal system. Public support swells after additional videos go viral, including a warning that ex-convict Jeon Seok-woo—who killed a pregnant woman in a drunk-driving incident—could be the next target. Despite public outrage, Seo's team is ordered to protect Jeon. They succeed in moving him to a safe house, where rookie officer Park Sun-woo, Jung Hae-in, stands out for his strength, politeness, and suspicious behavior.
Tensions rise as Sun-woo grows increasingly involved with the team. He meticulously secures the safe house, but Jeon escapes and is later found dead. Meanwhile, a staged confrontation on a live stream at Namsan goes wrong when Sun-woo brutally assaults a man he wrongly believes to be Haechi. The man turns out to be an actor hired by Editor Justice for content. As the real investigation continues, suspicion begins to fall on Sun-woo, especially after more signs point to someone with martial arts skills and knowledge of police operations.
Another suspect, Min Kang-hoon, a traumatized former soldier, emerges after killing a biker gang with his car. After a violent chase and rooftop fight, Kang-hoon is nearly killed by Sun-woo using an extreme jiu-jitsu hold, but the police intervene just in time. Though Kang-hoon is arrested, forensic evidence suggests Jeon was killed by someone using leg-based choking techniques. Seo recalls a video of Sun-woo using that same move, leading him to suspect that Haechi has been inside his own team all along.
As Seo investigates, Sun-woo kidnaps Editor Justice and sets a trap, forcing Seo to choose between saving his son or another innocent woman. The detective team rescues Seo's son just in time. Seo then confronts Sun-woo in a brutal final showdown, where Sun-woo nearly kills him before Seo turns the tables. As paramedics arrive, Seo revives him, insisting he live to face justice. In a quiet moment afterward, Seo shares a tender moment with his son, acknowledging the emotional distance of the past.
A post-credits scene reveals that Sun-woo has escaped while being transported to the prison.
