looking for the lost hikers. Her reluctant guide is hard-drinking Paul Grogan (BRADFORD DILLMAN), a reclusive outdoorsman whose recent divorce has left him with a deep distrust of people in general and women in particular. Together they explore the wooded hills until they come upon the army site. Could the couple have drowned in the pool? The discovery of an initialed locket leads Maggie to look for a way to empty the murky pool, but as she throws the drain lever a wild man leaps from the shadows to stop her. In the ensuing brawl the stranger is knocked unconscious. The empty pool, its contents flushed into the river, reveals only a dog skeleton.
It's back to square one for Paul and Maggie, but their assailant needs medical attention and the only authority within miles is the ranger station near the dam. Paul's suspicion that their "prisoner" is indeed responsible for the two hikers' disappearance prompts him to pilot his lashed-log raft down the river to the ranger station.
Floating down the river, Paul, Maggie and the stranger are greeted by a grisly scene. They discover the bloated corpse of Paul's friend Jack (KEENAN WYNN), his lower legs chewed to the bone.
Under Paul's relentless prodding, their attacker spills his strange story. He is Dr. Robert Hoak (KEVIN McCARTHY), a research scientist commissioned by the U.S. Army in 1965 to create a new kind of secret weapon for use against the North Vietnamese: a mutant strain of the underwater man-eater Piranha, able to live in both fresh and salt water environments of any temperature. The project was officially abandoned at war's end, but Hoak continued his experiments in secret until, by sheer number and ferocity, his killer-creations got hopelessly out of hand, breeding uncontrollably. Now hundreds of these deadly carnivores have escaped into the river! Paul and Maggie must reach the dam downriver before the floodgates are opened and the Piranha spill into the lake, by now filled with vacationers. And with Paul's own small daughter Suzy (SHANNON COLLINS) enrolled at the nearby camp, he hasn't a moment to lose.
Before reaching the dam they come upon a boy (JACK PAULESON) stranded atop a sinking canoe, encircled by Piranha.
Just as the boy is about to slip toward the surface, the repentant Hoak jumps into the river to save him and is lost in a churning of blood-red foam as his own razor-teethed creations literally tear him apart. The blood-crazed Piranha then attack the raft itself, chewing apart the very lashings to get at the warm bodies of Paul and Maggie. As the raft splits they manage to pole it to shore safely. With barely seconds to spare, Paul runs to the dam and delays the parting of the floodgates.
A convoy of military vehicles arrives at the scene soon after, summoned by the ranger. Colonel Waxman (BRUCE GORDON), government scientist Dr. Mengers (BARBARA STEELE) and a platoon of soldiers seal off the area and assess the danger. Convinced they have the Piranha bottled up behind the dam, Waxman orders the water poisoned with deadly chemicals. The crisis is past, he claims. But is it?
Paul points out that the Piranha might conceivably have swum back upriver into any of the small streams that branch away from the river and rejoin the lake below the dam, thereby escaping the poison. Waxman, fearing exposure of the secret project and the possible loss of his private financial stake in the resort, orders Paul and Maggie put under house arrest.
Escaping, Paul and Maggie steal a jeep and try to warn the owner of the summer camp, Dumont (PAUL BARTEL), by phone, but he dismisses the call as another of Paul's drunken ramblings. Waxman has Paul and Maggie arrested by an ambitious trooper, (BARRY BROWN), but again they manage to escape and proceed directly to the summer camp, this time in a police car.
Arriving at the camp, Paul is able to save his daughter but is too late to prevent tragedy: an innocent tube race becomes a nightmare when the Piranha attack and several children as well as a counselor (BELINDA BALASKI) are lost.
At the resort, recreational fun turns to terror as the Piranha attack swimmers, boaters and waterskiers, the clear river waters now running red with blood. Mass panic ensues, with Paul and Maggie arriving right in the middle of it. They make their way through an avalanche of twisted, wounded and dying victims to a motorboat. Paul's idea is to unleash the poisonous wastes stored in the smelting tanks of a nearby copper plant into the lake, pollutants so vile and deadly that nothing could survive them. The tanks themselves, it turns out, are now underwater, flooded when the level of the lake was raised. So Paul must go into the infested waters to reach the main valve ...