It is about a school of killer piranhas descending upon the bustling Lost River Lake Resort.

During the night, Barbara (Lorissa McComas) and her boyfriend David (Richard Israel) sneak into a closed down US Army test site, and discover a pool.

Maggie searches the area for any possible witnesses, eventually stopping by local homeowner Paul Grogan (William Katt), asking for any knowledge of the girl's disappearance.

Later that night, she wakes up and informs them that a school of piranha lived in the pool that they had drained, and are assumed to be headed to the river.

Darlene tries to save Laura (Soleil Moon Frye) but she falls in and the piranha kills her.

Maggie and Paul take a speedboat to the latter's old workplace, to open the valve containing toxins and spread them into the lake, in attempts to kill the piranha.

Upon arrival, the control room is flooded, and Paul must swim under to it and release the valve while Maggie stays in the boat counting to 200 before pulling him out.