Bradford Dillman stars as an antisocial mountain man and Heather Menzies as a rookie detective who race a school of mutant piranha downriver. The piranha are well ahead of them, and they kill several people on their way downstream. There's more than enough blood and gore for this drive-in meat market: a kids' summer camp and a waterfront amusement park await the little beasties. RWhen they try to warn the camp director and resort owner of the danger, they are arrested. Thanks to the woman's ingenuity they escape custody and race down to the camp in a state police car to warn everyone. But the piranha have already struck - and there are others who want to keep the danger of the piranha a secret!