Lou Garou is an alcoholic cop in the quiet town of Woodhaven who spends most of his days either dozing at the station or nursing drinks at Jessica's bar. When his friend Willie Higgins phones in a concern about occult activity near his gun store, the town’s police chief orders Garou to check it out. Garou brushes off Higgins’s alarm at first, chalking it up to the behavior of heavy metal fans, but a second disturbance compels the chief to insist on a proper investigation.
At the scene, Garou uncovers a chilling scene: occultists mid-ritual, attempting to sacrifice an upstart reformist politician who had been campaigning on anti-corruption. Garou is knocked unconscious and awakens the next morning in his own bed, bearing a pentagram carved into his stomach. From that moment, his senses sharpen dramatically and his wounds heal at an unsettling speed. As he digs into the case, he startles Jessica and his colleagues, who had dismissed him as lazy and incompetent, and begins to piece together a larger, more dangerous puzzle.
Back at the bar, Jessica taunts him and, in a private moment, nudges him toward drink. However, danger strikes again when two criminals from a local gang—roaming in pig masks—break into the bathroom and attack him. Garou, in the midst of transforming into a werewolf, fights back fiercely, killing one assailant and driving off the other. The gang’s leader retaliates by blinding one of their own when he claims to have seen Garou become a monster. The action spills over to Higgins’s home, where Garou is found handcuffed to a bed; Higgins explains that he had captured him for his own safety and to study what is happening.
Through careful research, they discover that the occult ceremonies in which a werewolf is sacrificed can amplify the power of reptilian shapeshifters who secretly run the town. The detective pair—Garou and his partner, Tina—investigate a spate of bar deaths and related armed robberies carried out by the pig-mask gang. Higgins urges Garou to stay restrained at night, and Garou agrees to be jailed in town for his own protection, even as his condition grows more volatile.
When a call for help hits the station, Garou—now fully transformed into a werewolf but retaining his human intellect—puts on his police uniform and heads to a nearby supermarket where the pig-mask gang has taken hostages. He dispatches the robbers with brutal efficiency and sets his sights on a meth lab, with Higgins cowering in the car. After returning to the station, Jessica attempts to seduce him while he is still in werewolf form, only to reveal that she is a shapeshifter and, in fact, the town’s elderly mayor, Mayor Bradley. Higgins discloses that he too is a shapeshifter and suggests that Garou’s alcoholism has left him stronger than their prior victims.
Drugged and led to a second ceremony, Garou faces a pivotal test: he is to be sacrificed during an eclipse to maintain the town’s hidden reptilian ruling elite. Tina arrives to confront the police chief, who shapeshifts into the gang leader—and Higgins—revealing their true identities. Tina and Garou kill Higgins, Jessica, and the gang members, but the chief strikes back with a final, brutal sneak attack and confesses to having killed his own father. Weakened as the eclipse approaches, Garou is shot by the chief but discovers that alcohol is the source of his enhanced power. He gulps down a flask, Tina impales the chief on a sword, and he uses the weapon to stab Tina in a last-ditch struggle. In the end, Garou shoots and kills the police chief, and he and Tina stumble away together, with Garou promising to get Tina to a hospital after he secures another drink.
