When two divers mysteriously disappear in the waters near a secretive military facility, insurance investigator Maggie McKeown is dispatched to find out what happened. She enlists the help of an alcoholic recluse named Paul Grogan, who knows the area's waterways. They come across the abandoned facility and, believing it has something to do with the divers' disappearance, they drain the pool to investigate further. Unbeknownst to them, by doing so, they unwittingly release genetically engineered, carnivorous piranhas into a river leading to a nearby summer resort and water park.

As the lethal fish swim downstream, they attack anyone in their path, causing chaos and bloodshed. Maggie and Paul realize the gravity of their mistake and set off on a desperate quest to prevent further attacks. Racing against time, they attempt to warn the unsuspecting vacationers and the management at the water park of the impending danger. With the local dam scheduled for its annual opening to allow for extra summer recreation, the window to stop the piranha from reaching an even larger body of water and endangering more lives is rapidly closing.