While searching for missing teenagers, novice skip tracer Maggie McKeown (Heather Menzies) and one of the locals, town boozer Paul Grogan (Bradford Dillman) stumble upon a top-secret Army laboratory doing genetic research on piranha fish for the purpose of developing biological warfare. When they are accidentally released from the compound, the deadly school of eating machines head downstream and eat everything, and anything, in their path. Just when you thought it was safe to go in the river ...