The film stars Bradford Dillman, Heather Menzies, Kevin McCarthy, Keenan Wynn, Barbara Steele, and Dick Miller.

It tells the story of a river being infested by lethal, genetically altered piranha, threatening the lives of the local inhabitants and the visitors to a nearby summer resort.

Executive produced by Roger Corman, Piranha is the first installment in a series of low-budget B movies inspired by the film Jaws (1975), which had been a major success for Universal Pictures and director Steven Spielberg.

Sometime later, determined yet absent-minded skiptracer Maggie McKeown is dispatched to find the missing teenagers near Lost River Lake and hires a surly backwoods drunkard, Paul Grogan, to guide her.

Locating a drainage switch for the nearby pool, Maggie empties it to search the bottom, but a haggard man enters and tries to stop her until Paul subdues him.

The stranger reveals himself as Dr. Robert Hoak, lead scientist of a defunct Vietnam War project called Operation: Razorteeth, which involved genetically engineering a ravenous and prodigious strain of piranha that could endure the North Vietnamese rivers' cold waters and inhibit Viet Cong movement.

Paul realizes that if the local dam is opened, the piranha will gain access to the Lost River resort and the nearby summer camp.

A military team led by Colonel Waxman and former Razorteeth scientist Dr. Mengers spread poison upstream, ignoring protests that the piranha survived the first attempt.

Intending to prevent the piranha from reaching the ocean and spreading around the world, Paul and Maggie commandeer a speedboat and rush to a shuttered smelting plant in the hopes that they can use its industrial waste to kill the mutants.

[9] On March 13, 2015, the film screened in a R-rated Ultimate Edition presented by Fangoria magazine and special guests Rebekah McKendry, curator Mike Williamson and actress Belinda Balaski.

Roger Ebert mocked the "really bad special effects" and the "odd compulsion" of the characters "to jump into the water the very moment they discover it is infested by piranhas".

[16] Screen International's Marjorie Bilbow wrote that the film "works as spirited scream and giggle nonsense because there is always something happening and Joe Dante directs with a slyly tongue-in-cheek awareness of the existence of big brother Jaws".

Aja explained, "My goal is not to remake Piranha, but to create a completely new adventure paying homage to all the creature films [...] I am very proud to follow the path of Joe Dante and James Cameron, and look forward to working with Greg Levasseur to write, produce, and direct such a fun and gory thrill ride.