In Aroostook County, Maine, Marine Fish and Game officer Walt Lawson is scuba diving in Black Lake when he is suddenly attacked and bitten in half by an unknown creature.
Weeks later, Sheriff Hank Keough, Fish and Game officer Jack Wells and American Museum of Natural History paleontologist Kelly Scott go to the lake to investigate the incident with mythology professor/enthusiast Hector Cyr joining them. A series of strange events soon ensue, including Kelly and Hank's canoe mysteriously being pushed into the air and flipping over, the discovery of a severed toe as well as a severed moose head before finally the decapitation of Burke, one of Hank's deputies.
Later, as Hank and Hector argue, a large black bear attacks them, but a gigantic saltwater crocodile suddenly leaps out of the water, snatches the animal in its jaws, and drags it into the lake. After finding Burke's severed head, Jack, Kelly, and Hank witness Delores Bickerman, an elderly hermit living near the lake, feeding a blindfolded cow to the giant crocodile. She reveals that she has been feeding the reptile for years after the crocodile followed her husband home and eventually killed him. Afterwards, she was placed under house arrest for initially lying to the police.
Hector decides to take Deputy Sharon Gare on a trip in his helicopter and unexpectedly lands it in the crocodile's territory where it lives. While he is scuba diving, he is confronted by the creature, but he and Gare escape after distracting it with an inflatable raft. Later, Jack and Hank plan to allow Maine Fish and Game to kill the crocodile when they arrive, but Hector suggests instead that he should lure it out of the water and drug it into unconsciousness. Jack reluctantly accepts the proposal and they use one of Bickerman's cattle, dangled from the helicopter, as bait.
After a few hours, the crocodile soon appears and rears up as it lunges at its prey. Hector pulls up and loses the animal, but crashes the helicopter into the lake. The crocodile comes on land and begins to attack Jack, Kelly and the group. Kelly is knocked into the lake by the crocodile's tail, but she makes it into the helicopter in time. The crocodile catches up to Kelly and attacks again, but is itself trapped in the helicopter. Jack grabs a gun and shoots it, but the firearm turns out to be a tranquilizer rifle. As Hector comes out of the water, another crocodile attacks and bites him, but Hank blows it up with his grenade launcher. Soon after, Maine Fish and Game officers arrive, where they load the neutralized crocodile onto a truck and take it to Portland, Maine to figure out what to do with it.
One week later, Bickerman is shown feeding bread crumbs to many baby crocodiles, revealing the two adults were actually a mating pair. The surviving adult crocodile is later seen tied to the back of a flat-bed truck, speeding down a road to an unknown location.
