At Dark Waters, a Native American sacred land containing an enigmatic swamp spirit, a teenager is murdered by a plant-like monster. The swamp used to be a place of beauty, but all that changed once men started drilling activities on swamp land. The oil drill on the swamp is owned by the Schist company. The Native Americans believes that the swamp is fighting back. Just next to the swamp are the oil storage facilities for the Schist company.The following day, young replacement sheriff Kyle Williams (Matthew Le Nevez) reaches Bywater and meets with deputy sheriff Fraser (Alex O'Loughlin), who tells him the previous sheriff Jim Corely is among 47 missing persons since oil tycoon Fred Schist (Jack Thompson) bought the ancient tribal lands from shaman and Seminole chieftain Ted Sallis (Mark Stevens), the first to disappear. Jake (Pat Thompson) is Fred's son. Teri Richards is a local activist who protests Schist's commercial drilling activities in the swamp and argues that the activities violate tribal law. She also says that tribe never saw any of Schist's money as Sallis took all of it and ran.Schist claimed Sallis had sold legally and escaped with the money and asked the sheriff for help: Local protesters opposed his perfectly legal activities, and Mestizo scoundrel Renee Laroque (Steve Bastoni) was sabotaging his facilities. Fraser also tells Williams that Corely believed that Laroque was working with Sallis to drive Schist away from the swamp. Williams investigated this while trying to find an explanation for the missing people, some of which were found brutally murdered with plants growing from inside their bodies. Photographer Mike Ploog (Robert Mammone) and shaman Pete Horn (Rawiri Paratene) tell Williams local legends about the guardian spirit, suggesting that it could be real.Schist complains about vandalism of his equipment in and near the swamp that is putting him months behind schedule and causing huge losses. As sabotage and murder continue, Williams investigates the swamp with Fraser and finds the previous sheriff's corpse. Medical examiner Val Mayerik (Brett Leonard) admits that the previous sheriff had ordered him to file the deaths as alligator attacks, even if Mayerik believed otherwise.Williams suspects that Laroque is involved in the murders. Williams and Fraser try to track Laroque, who lives in the swamp, by using a canoe. At the same time, Schist sends local thugs the Thibadeux brothers Wayne (John Batchelor) and Rodney (Ian Bliss) to track and murder Laroque. The monster in the swamp finds the Thibadeux and kills them. Williams is ensnared by Laroque, who admits having helped Schist buy the lands, but then claims that Sallis was opposed to the sale; Laroque insists that the guardian spirit would keep on murdering until Schists stops desecrating the sacred swamp. Fraser tries to arrest Laroque, but the Man-Thing timely appears and murders Fraser; Laroque knocks Williams down and escapes. Williams wakes up and finds Ploog, who has blurry pictures of the monster; the sheriff seizes the photographs and forbids Ploog to come back to the swamp.The following day, Williams interviews Horn and Schist, with schoolteacher Teri Richards' (Rachael Taylor) help, for whom he starts having romantic feelings. Horn goes to the swamp and tries to stop the Man-Thing with prayers and sacrificing his life but, although the monster kills Horn, he is not affected. That night, Mayerik autopsies the old sheriff and finds a bullet. He tries to tell Williams, but he is back at the swamp, unreachable; he tells Richards, and she goes to the swamp to tell Williams. Meanwhile, Ploog had returned to the swamp, trying to get a picture of the monster, but instead he startles Fred Schist, who was in the swamp to murder Laroque. Fred shoots and kills Ploog. Soon afterward, Laroque ambushes and defeats Schist's son Jak).Williams finds Ploog's corpse and reasons that Schist murdered Ploog. He then meets Richards, who tells him about Mayerik's autopsy. Williams concludes that Schist is guilty of several murders, trying to incriminate Laroque simply to avoid punishment - in fact, by Schist's confession to Laroque, he murdered Sallis and buried him in Dark Waters (which, due to the magic embedded in the soil, made him go back as the Man-Thing). Richards reveals that she can guide him to Laroque's lair, but the Man-Thing starts chasing them. He chases them to the drilling tower at Dark Waters, where Schist is leveling his weapon at Laroque in an attempt to prevent Laroque to blow it away with dynamite. Laroque nonetheless tries to detonate his bomb and is shot and wounded by Schist; Schist then wounds Williams.However, the Man-Thing arrives and brutally murders Schist by filling his body with oil. Then, the Man-Thing moves toward Williams and Richards. Laroque sacrifices himself shouting at the monster and blowing the bomb. The monster survives the flames, but then is absorbed back to the land, allowing Williams and Richards to leave unharmed.