Snakehead fish invade Cultus Lake in the small town of Cultus, Maryland, prompting the authorities to poison the lake, a drastic measure that seems to wipe out the threat. Two years later, bodies begin surfacing from the lake, and Sheriff Patrick James [Bruce Boxleitner] temporarily closes off the water as officials grow increasingly concerned about what is happening beneath the surface. After unseen predators claim Patrick’s daughter and take the lives of Amber James’s boyfriend James [Matthew MacCaull] and James’s friend, the town is left reeling. The investigation deepens when local doctor Doc Jenkins [William B. Davis] presents a tooth found among the bodies to the sheriff, a clue that hints this is no ordinary fish problem. Patrick tries to persuade the town’s mayor to shutter the lake for good, but the local economy—tied to tourism, fishing, and bait shops—limits how far he can push.
Marine biologist Lori Dale [Carol Alt] arrives to help Patrick unravel the mystery, and she quickly identifies the threat as unusually large snakeheads. She theorizes that something in the lake is triggering abnormal growth in these fish, a suspicion that grows darker as more deaths occur. Lori’s expertise leads to a shocking discovery: the snakeheads have been dosed with human growth hormones to make them bigger and more dangerous. A fisherman later captures one of the grotesquely enlarged snakeheads, reinforcing the mayor’s conviction that the lake isn’t worth closing—believing the captured specimen proves the danger is isolated. Yet the truth proves far more sinister. Patrick and Lori set out on the lake to locate the source of the hormones, while Amber [Chelan Simmons] rallies her friends—Jagger [Juliana Wimbles], Luke [Ryan McDonell], and Craig [Chad Krowchuk]—to help her kill the snakeheads in vengeance for the deaths they have caused.
On Craig’s boat, the group discovers a brutal reality: the snakeheads are cannibals that prey on juvenile fish as they grow monstrous in size. They trail a school of these predators, nearly colliding with two other fishermen as the fish lash out. The snakeheads strike, consuming Craig and one of the fishermen, while Luke is knocked overboard and Jagger accidentally destroys the other boat’s gas tank, blowing it up. Amber and Jagger end up stranded on a remote island in the middle of the lake after a crash, with no cell reception. They manage to find Luke alive, though badly injured, a grim reminder of how quickly the situation spirals out of control.
Back on land, Patrick and Deputy Reece breach Jenkins’ laboratory in a bid to uncover something incriminating. They find a receipt for a large shipment of human growth hormone and arrest Doc Jenkins [William B. Davis], who confesses to using the hormones to boost the lake’s fish population for Colin’s bait shop after the poisoning, not realizing the snakeheads were already thriving in the water. On the island, Amber, Jagger, and Luke locate a cabin where a half-devoured man lies inside, and Luke pushes further to find a snakehead feasting on a person nearby. The creature attacks Luke, wounding his leg, but Amber fights back and kills it with an axe.
Patrick learns that Amber is stranded on the lake’s expanse, and he and Lori rush to locate her. Lori brings an electric stun device to the mission, intending to use it against the remaining snakeheads. As the snakeheads ram their boat, Lori’s gadget disrupts the controls, forcing a crash into a dock on the island. The trio spots a giant, whale-sized snakehead looming in the murky water, a terrifying sign that the danger is far from over. Colin, intent on burying the hormone packages, digs a hole but is quickly dispatched by the very creatures he unleashed.
As the danger closes in on the cabin, the bodies of Amber, Jagger, and Luke become crucial in drawing Patrick and Lori toward the shoreline. The group makes their way to the electrical shed, hoping to weaponize the lake’s own current against the creatures. The plan hinges on cutting power and flooding the waters with electricity to toast the snakeheads. Lori ventures to rescue Amber and Jagger from the house, and the three fight off the remaining monsters before Patrick arrives with a decisive blow. He knocks the electric cable loose, and Lori hurries to drop the cable into the water, electrocuting the last of the snakeheads. In the end, the four survivors—Patrick, Lori, Amber, and Jagger—embrace as the fried snakehead bodies burn away, a grim but hopeful close to a deadly nightmare that began with poisoned water and spiraled into a battle for their town’s future and their lives.
