PIRANHA

Two teenagers find an abandoned military compound and skinny dip in a holding pool, but are attacked and killed by unseen creatures.

Sometime later, determined yet absent-minded skiptracer Maggie McKeown is dispatched to find the missing teenagers near Lost River Lake and hires a surly backwoods drunkard, Paul Grogan, to guide her. They discover the compound and find bizarre jarred specimens and evidence of an occupant inside. Locating a drainage switch for the nearby pool, Maggie empties it to search the bottom, but a haggard man enters and tries to stop her until Paul subdues him. The pair finds a skeleton in the pool's filtration trap and learn it was filled with saltwater. The man awakens and steals their jeep, but crashes due to being disoriented and is taken to Paul's home. The next day, as the three travel downriver, the stranger tells the pair that the pool was filled with a school of piranhas and Maggie released them into the river. She and Paul are skeptical until they come across the corpse of Paul's friend Jack.

The stranger reveals himself as Dr. Robert Hoak, lead scientist of a defunct Vietnam War project called Operation: Razorteeth, which involved genetically engineering a ravenous and prodigious strain of piranhas that could endure the North Vietnamese rivers' cold waters and inhibit Viet Cong movement. The project was canceled after the war ended, but the mutant specimens survived, and Hoak tended to them to salvage his work. Paul realizes that if the local dam is opened, the piranhas will gain access to the Lost River resort and the nearby summer camp. On their way, the trio rescue a boy whose father was killed by the piranhas, but the mutants kill Hoak before he can reveal how to kill them, and the survivors narrowly make it to shore. Grogan successfully stops the dam attendant from opening the spillway before calling the military.

A military team led by Colonel Waxman and former Razorteeth scientist Dr. Mengers spread poison upstream, ignoring protests that the piranha survived the first attempt. When Paul discovers a tributary that bypasses the dam, Waxman and Mengers quarantine him and Maggie to prevent them from alerting the media. The pair escapes, but Waxman alerts law enforcement to capture them while the piranhas attack the summer camp, injuring the children and killing Betsy, a camp counselor.

Waxman and Mengers arrive at the resort to intercept Paul and Maggie, but the piranhas attack and kill many vacationers and Waxman. Intending to prevent the piranha from reaching the ocean and spreading worldwide, Paul and Maggie commandeer a speedboat and rush to a shuttered smelting plant, hoping to use its industrial waste to kill the mutants. They arrive at the plant before the piranhas, but the control office is submerged, forcing Paul to go underwater with a safety line. Despite being attacked by the piranhas, he opens the valves, and Maggie pulls him to safety. The pair returns to the resort, where Paul enters a catatonic state. During a televised interview, Mengers spins an altered version of events and downplays the piranha's existence.

The piranhas attack the summer camp and water resort, causing chaos and casualties. Ultimately, efforts are made to stop the spread, but the threat is not fully contained.

Piranha ends on a note of uneasy resolution, with the military's attempts to poison the fish proving insufficient. The survival of the piranhas underscores the dangers of unchecked scientific experimentation and satirizes authority figures' inability to manage the disasters they create. The final moments leave the door open for the carnivorous threat to continue elsewhere.