Bradford Dillman stars as Paul Grogan, a reclusive alcoholic living in a cabin out in the woods in an undisclosed part of the United States. He's visited by a skiptracer named Maggie McKeown (Heather Menzies), who is out to find a couple of backpackers who have disappeared without a trace. Their search leads them to an abandoned Army research facility solely inhabited by a strange scientist, Dr. Hoak (Kevin McCarthy). Hoak has been keeping up the research on “Operation Razorteeth,” where mutant carnivorous piranha fish became bioweapons they can use to destroy the coldwater river systems of North Vietnam. The war ended, and the research stopped, but the scientist wouldn't abandon it. A mishap sees the piranha drained out into the nearby river, where they begin to wreak havoc on anyone unlucky enough to be in the water with them. Joe Dante directs this Roger Corman production from a script by John Sayles.