Piranha

Barbara and David sneak into the closed US Army test site and discover a pool. They swim and are suddenly attacked and killed by an unseen force. The incident sets off the investigation that follows.

The next day, J.R. Randolph hires private investigator Maggie McNamara to look into Barbara's disappearance, believing she may be a runaway. Maggie accepts the case and begins to follow leads in the area. The motive is to uncover the truth and clear up the mystery.

Maggie, guided by Paul Grogan, visits the army test site to search for witnesses. They are led to the pool, and decide to drain it to look for evidence. A confrontation looms with Dr Leticia Baines.

While draining the pool, Dr. Leticia Baines attacks Maggie and Paul to stop them, but they persevere. They drain the water and uncover a skeleton at the bottom. Baines steals the jeep and crashes after losing consciousness.

Leticia regains consciousness and informs Maggie and Paul that a school of piranha lived in the drained pool and are now headed toward the river.

Paul learns his daughter Susie is downstream at a scout camp and joins Maggie to confront Randolph about delaying the resort's grand opening. They attempt to persuade him, but he remains unconvinced and refuses to halt the plan. The pair then races downstream to warn those at risk.

Maggie and Paul are arrested after Randolph dismisses their warnings as nonsense. They manage to escape custody to continue warning people downstream about the danger.

The piranha reach the camp and attack the kids. Susie uses a raft to rescue her friend Darlene. Darlene tries to help Laura, but Laura is pulled under and killed. Maggie and Paul finally reach the camp and begin to intervene.

Maggie calls the resort to warn them, but the staff ignores the warning. She and Paul drive to the resort themselves but arrive too late; most swimmers have already been killed and the resort braces for disaster.

Randolph realizes his decisions may lead to legal action and commits suicide to avoid consequences. The town reels from the catastrophe, dealing with the loss and fallout.

The mayor proclaims the piranha are dead, but Maggie and Paul decide to release toxins by opening a valve at Paul's old workplace. They fight through a flooded control room to reach the valve.

Paul swims to the valve, releases it, and toxins spread through the lake toward the piranha. Maggie starts the boat engines and pulls Paul away from the school. She discovers the rope has been cut, making her fear Paul did not survive—until he surfaces from the water, gravely wounded but alive.

In the aftermath, the town is told the piranha are dead, but the trilling sounds of piranha are heard from the ocean, revealing that many survived and swam away. The town braces for further danger.