David (Roger Richman](/actor/roger-richman)) and Barbara (Janie Squire) are adventurous young backpackers who stumble upon what appears to be an abandoned military outpost. Intrigued, they come across a vast, man-made reservoir, and decide to indulge in some skinny dipping. However, their fun takes a horrifying turn when they are suddenly attacked by an unknown underwater entity. Their blood-curdling screams echo through the facility, alerting someone still present inside the military complex.
Enter Maggie (Heather Menzies-Urich](/actor/heather-menziesurich)), a determined investigator employed by Barbara's parents to uncover the fate of their daughter and her partner. Fueled by funding from her boss Earl (ichard Deacon), Maggie journeys to the remote mountainous area where David and Barbara were last seen. She seeks out Paul Grogan ([Bradford Dillman), a rugged survivalist residing in a log cabin with only his young daughter for company, as she accompanies him to the military facility. Upon their arrival at the pond, Paul notices something unusual: the water has a salty quality. As they venture into the complex, they discover a laboratory filled with bizarre creatures, including a nightmarish, lizard-like beast.
Maggie theorizes that David and Barbara could have drowned in the pond and decides to drain it to investigate further. As she flips the switch, chaos ensues when a frantic white-haired man (Kevin McCarthy) appears, desperately trying to deactivate the drain. In the chaos, he is knocked out cold. After draining the pond, Paul and Maggie are horrified to uncover the skeletal remains of David and Barbara caught in the drain.
Their harrowing encounter with the man continues when he attempts to hijack Maggie’s jeep but crashes it, injuring himself in the process. They take him back to Paul’s cabin for the night, where he rambles incoherently about "razorteeth" and warns that danger is imminent.
The next day, they transport the injured man downriver on a makeshift log raft. Upon waking, he reveals himself to be Dr. Hoak, a former military scientist, and with grim urgency, he explains the horrific truth: the pond housed a genetically modified species of mutant piranha, initially created for use in the Vietnam War. After the war, the project was shut down, but Hoak, driven by curiosity, continued his sinister experiments.
The mutant piranha begin their deadly rampage without delay. Tragedy strikes when Jack (Keenan Wynn), a friend of Paul’s, succumbs to the ruthless fish while fishing; his screams are drowned out by the bloodshed. In another chilling scene, a father fishing with his son meets a gruesome end, dragged underwater and devoured by the piranha, while the terrified boy clings desperately to the capsizing boat.
Concern mounts for Paul’s daughter, Suzie (Shannon Collins), who is at a summer camp situated downstream from the approaching menace. With the camp also close to a new resort poised to open, Paul and Maggie scramble to warn both locations, but their pleadings are met with disbelief and dismissal.
As the military becomes aware of the piranha threat, they dispatch Dr. Mengers (Barbara Steele) and a team of soldiers to assert control. Initially, Mengers scoffs at the idea of the piranha, but Maggie soon discovers that Mengers has a personal history with Dr. Hoak, leading her to suspect a cover-up by the military. The duo are subsequently detained, preventing Paul from reaching out to his daughter or making a known warning.
In a twist of fate, they manage to escape but are quickly caught by local authorities. Paul does manage to relay an urgent message to the camp's head counselor (Paul Bartel), but the allowance of alcohol tarnishes his credibility.
As chaos unfolds at the summer camp, Suzie, terrified of the water, finds herself confronted with a frantic scene as counselors (Belinda Balaski and Melody Thomas Scott) panic as they try to save other children. In a brave act, Suzie ventures into the water to attempt a rescue, but tragedy strikes when one of her counselors is devoured by the relentless piranha.
Paul and Maggie arrive all too late, but after ensuring Suzie's safety, they race towards the resort. The owner, Buck Gardner (Dick Miller), rebuffs their calls, steeped in the advice of military authorities. While on the verge of despair, catastrophe strikes when the piranha stage a mass attack at the resort, leading to shocking casualties and pandemonium.
During their frantic journey, Paul recalls the originally salty water of the test pond, realizing that Hoak's altered piranhas can survive in both salt and fresh water. If unchecked, they would absolutely contaminate all water sources, including the ocean.
In a race against time, Paul and Maggie commandeer a speedboat, seeking to thwart the piranha by introducing pollution from a nearby sewage facility. However, upon arrival, they find the plant submerged, leaving Paul no choice but to swim underwater to unleash the sewage. As he bravely dives in, danger looms when piranha close in on him. Maggie counts down, ready to pull him to safety if needed.
Barely managing to escape, Paul is gravely wounded but alive. With Maggie and Suzie assisting him as medics attend to numerous injuries at the resort, the story takes a critical turn as Dr. Mengers addresses the media, claiming the threat from the piranha has been neutralized. Yet, lingering questions remain: have the piranha truly been eradicated, or do they still pose a hidden threat to the world’s waters?
