Barbara, Lorissa McComas and her boyfriend David slip into a closed-down US Army test site one night and discover a pool that seems ordinary at first. They swim for a thrill, but an unseen force attacks them in the water, leaving Barbara dead and David killed in the shadows—an ominous start that hints at something far more dangerous lurking beneath the surface.
The next day, J.R. Randolph, [Monte Markham], Barbara’s uncle, hires private investigator Maggie McNamara [Alexandra Paul] to look into the incident, suspecting that Barbara may have been a runaway rather than simply missing. Maggie begins sifting through the area, speaking with locals and gathering fragments of the truth that could connect the tragedy to a larger danger.
Her search leads her to local homeowner Paul Grogan, [William Katt], whom she questions about any sighting or information related to Barbara’s disappearance. He denies knowing anything concrete but directs Maggie toward the army test site and the pool, offering what little detail he can remember. The pair braves the eerie facility, hoping to uncover clues that might explain the inexplicable.
As Maggie and Paul press on, Maggie argues that draining the pool could reveal hidden evidence. They proceed with the draining, but their plans are interrupted when Dr. Leticia Baines, [Darleen Carr], unexpectedly confronts them and tries to stop the operation. Her interference is aggressive, but she is eventually overwhelmed by the draining process. Down below, Maggie and Paul uncover what they believe to be a dog’s skeleton, a clue that deepens the mystery rather than resolving it. In a bold move, Baines steals a jeep and flees, only to crash and lose consciousness. When she wakes later, she reveals a startling claim: a school of piranha lived in the pool they drained and may be headed toward the river.
Paul, aware that his daughter Susie, [Mila Kunis], was at a scout camp downstream, joins Maggie to confront Randolph and advocate shutting down a grand resort opening downstream. Together, they confront the political roadblocks and bureaucratic resistance that stand in the way of protecting the downstream communities, and they navigate a series of twists and turns to try to save people who might be in harm’s way.
Their efforts are complicated when Randolph arrests Maggie and Paul after dismissing the piranha warning as nonsense. They manage to escape custody, intent on warning the residents of Lost River before the danger can strike at full force.
The threat proves real as the piranha descend on the camp, where Susie shows courage by grabbing a raft and helping a friend escape. Laura, played by Soleil Moon Frye, falls into the water and is pulled under by the terrifying school of fish. Maggie and Paul reach the camp in time to see the chaos unfold; Paul leaps into a canoe to save the children, while Maggie makes calls to the resort to warn them, only to find the warnings ignored. Undeterred, the two speed toward the resort, but arrive too late—the pool of carnivorous fish has already claimed many swimmers. Randolph finally recognizes the gravity of his miscalculations.
Desperate to curb the threat, Maggie and Paul race to Paul’s old workplace to open a valve that would release toxins into the lake. They reach the control room, but the facility is flooded. Paul must swim under the water to activate the valve, while Maggie stays aboard the boat, counting to 200 before pulling him to safety. After the valve is opened and toxins begin to spread, the piranha attack Paul as Maggie fights to pull him away. He survives the strike but is gravely wounded, and the two share a precarious moment of relief as he surfaces.
In the aftermath, Randolph, overwhelmed by the consequences of his choices, commits suicide to avoid legal accountability. The town’s mayor announces that the piranha are thought to be dead, offering a fragile sense of closure. Yet the film closes with a haunting reminder: the ocean holds the possibility that many piranha have survived and escaped, lurking beyond the shore.
