In the remote wilderness of Aroostook County, Maine, a shocking incident unfolds as Fish and Game officer Walt Lawson is scuba diving in Black Lake. Suddenly, he becomes the victim of a vicious attack, meeting a horrifying fate when he is bitten in half by an unidentified creature lurking beneath the surface.
The following day, Sheriff Hank Keough, along with fellow officer Jack Wells and paleontologist Kelly Scott, from the American Museum of Natural History, heads to the lake to investigate this bizarre occurrence. Joining them is Hector Cyr, a mythology professor and crocodile aficionado. Their investigation takes a dark turn when, after an accident capsizes their canoe, they stumble upon a severed human toe and a mutilated moose head. The tension escalates as Deputy Burke falls prey to the creature while out on a boat, meeting a gruesome end when his head is violently bitten off.
As Hank and Hector bicker amidst the growing danger, the situation intensifies when a massive 30-foot long saltwater crocodile emerges from the depths, snatching a bear right before their eyes and dragging it into the lake. Tensions rise further when the group finds Burke's severed head, leading them to encounter Delores Bickerman, a reclusive old woman living near the lake. She reveals her shocking past: for years, she has been feeding the crocodile, which first appeared when it followed her late husband Bernie home, only to later kill him after he intervened in a frantic horse chase. Delores's initial deception to the authorities had resulted in her house arrest.
In a reckless bid to confront the crocodile, Hector takes Deputy Sharon Gare on a helicopter venture, accidentally landing in the creature's territory. While scuba diving, Hector encounters the beast but manages to escape with Gare, thanks to some clever distraction tactics. Meanwhile, Jack and Hank devise a plan to wait for the Florida Fish and Game team to kill the crocodile. However, Hector proposes a more risky alternative: he will lure the creature out with one of Delores's cattle as bait for tranquilization.
As the plan unfolds, Hector struggles to keep the animal steady during the descent, ultimately crashing their helicopter into the lake. In the chaos, the crocodile pursues Jack, Kelly, and the others onto land. Despite a near-fatal encounter, Kelly manages to reach safety in the helicopter, only to find the crocodile cornering her. In a twist, Jack mistakenly shoots the creature with a tranquilizer instead of a lethal weapon. Things take a further turn when another crocodile emerges, attacking Hector, but Hank heroically intervenes with a grenade launcher, destroying the threat.
As the ordeal comes to an end, officials from Florida Fish and Game arrive to transport the subdued crocodile to Portland, Maine, intent on determining its fate. The story wraps up one week later with a shocking revelation: Delores is shown feeding breadcrumbs to numerous baby crocodiles, hinting that the two adult crocodiles were, in fact, a mating pair. The surviving adult is then seen secured to a truck, speeding down a road, leaving the audience to wonder what happens next.
