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. They go swimming, but are attacked and killed by an unseen force in the pool. The next day, J.R. Randolph (
Monte Markham) the uncle of Barbara, hires
private investigator Maggie McNamara (
Alexandra Paul), to investigate the incident, believing her to be a
runaway. 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. He claims to have not seen her, but leads her to the army test site where they discover the pool. They enter to look for any clues, until Maggie thinks they should drain the pool. As she starts the draining, a scientist named Dr. Leticia Baines (
Darleen Carr), encounters them and attacks Maggie and Paul to stop the draining, but is too late to do so. They investigate the bottom of the pool and discover a
skeleton, which they believe is that of a
dog. Baines steals the
jeep, but crashes after losing consciousness. 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. Paul, knowing his daughter Susie (
Mila Kunis) was at a
scout camp just
downstream of the river joins Maggie and Paul to visit Randolph and try to convince him to shut down a grand opening of a
resort just downstream. They fail, and have to make many twists and turns to try and save people downstream. On the way to warn people of the piranha, Maggie and Paul are arrested after Randolph had claimed that they lied about the piranha, dismissing it as "nonsense". They eventually escape from
custody to warn the people of Lost River. The piranha first make their way to the camp, attacking the kids. Susie takes a
raft and saves her friend Darlene. Susie tries to save Laura (Soleil Moon Frye) but she falls in and the piranha kills her. Maggie and Paul make it to the camp. Paul grabs a
canoe and saves his daughter and the kids. Maggie calls the resort to warn them of the danger but is ignored. She and Paul drive to the resort themselves but arrive too late; the school of piranha having killed most of the swimmers. Randolph now sees and realizes his mistake. 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. The piranha attacks Paul but he successfully releases the valve, spreading the toxins. Maggie starts the boat's engines, pulling Paul away from the piranha school. As Maggie pulls out the
rope, she discovers that it was cut loose, making her think that Paul did not make it until he surfaces from the water, gravely wounded but still alive. In the aftermath, J.R. Randolph commits
suicide to prevent himself from facing
legal action. After the horrific incident, the Mayor of Lost River announces that the piranha somehow are all dead, but then at the
ocean the trilling sounds of the piranha are heard, revealing that a significant number of piranha have survived and escaped into the ocean. ==Cast==