Fisherman Matthew Boyd (Richard Dreyfuss) fishes in Lake Victoria, Arizona. He whistles, Show Me the Way to Go Home (an homage to 'Jaws'). A huge earthquake shakes the lake, and opens a massive underwater trench, creating a whirlpool beneath Matt's boat. Thousands of piranha fish pour out of the newly opened chasm. Matt falls overboard into the whirlpool, and is violently eaten by the school of piranha, leaving Matt's mangled body behind.
Meanwhile, spring break descends upon Lake Victoria, bringing thousands of college students to the area. Lake Victoria Sheriff Julie Forester (Elisabeth Shue) is having a difficult time juggling drunken college students, recent earthquake reports, and her normal duties. Her 17-year-old son, Jake (Steven R. McQueen) looks on as his mother arrests a particularly rowdy frat-boy. Jake takes in the spring break party atmosphere as he goes to pick up his little 10-year-old sister, Laura (Brooklynn Proulx) from music class. On the way Jake crosses paths with Kelly (Jessica Szohr), a local girl for whom he harbors a huge crush. Kelly invites Jake to come to a backstage party at a concert, with tickets procured by the local bully Todd. Todd and his friend arrive. They see Kelly talking with Jake, throw a slushy at Jake's shirt, ruining it, and give him the finger. Feeling awkward, Kelly leaves with Todd. Jake goes to find Laura. Laura talks with Danni (Kelly Brook), a local Girls Gone Wild-ish glamour model in town to shoot a video with her boss and porn director Derrick Jones (Jerry O'Connell). Jake arrives, and is reluctantly enlisted to be Derrick's location scout, due to his familiarity with the area. Derrick offers to pay Jake for a day's work and Jake accepts.
That night Julie and Deputy Fallon (Ving Rhames) are called out to investigate the disappearance of Matthew Boyd. They arrive at his fishing spot; discover his empty boat, and ultimately his mangled corpse. They consider shutting down the lake. At home, Jake learns about Derrick Jones from a porn-site, and is interrupted by his mother, Julie, who orders him to stay at home the next day, to babysit his little brother Zane (Sage Ryan), and sister Laura. Jake agrees to stay home and watch the kids tomorrow.
The next day Julie and Fallon learn that in addition to patrolling spring breakers, they must accompany a team of seismologists to investigate the newly opened chasm in the lake caused by the earthquake. Team leader Novak (Adam Scott), and divers Sam (Ricardo Chavira) and Paula (Dina Meyer) take Julie's patrol boat out to the chasm. Sam and Paula swim down into the chasm. Paula waits at the chasm opening, to maintain radio contact, as Sam swims into the massive underwater lake. There, Sam discovers tens of thousands of unhatched piranha eggs. Sam is suddenly bitten, and bleeds into the water. The piranhas appear and eat Sam. Novak jumps in after them. He attempts to save Paula, who is swiftly eaten. Bringing Paula aboard, Novak and Julie discover a Piranha, which had attached itself to her corpse, flopping around on the deck. They trap the fish, and immediately take it to local Pet Shop owner, Mr. Goodman (Christopher Lloyd). Goodman identifies the piranha as an extremely violent, cannibalistic species which went extinct two million years earlier. They decide to evacuate Lake Victoria.
Meanwhile, Jake shirks his babysitting duties, and agrees to pay Laura and Zane $60 in exchange for lying to their Mom and staying in the house. Jake makes his way to Derrick's boat, and watches Danni and another model named Crystal (Riley Steele) dance provocatively on the deck. As Jake is about to board, he runs into Kelly, who confesses that Todd lied about having back stage passes, and asks why he isn't babysitting his siblings. Derrick interrupts their conversation, being both inebriated and attracted to Kelly, he invites her aboard the party boat. Kelly agrees, to Jake's annoyance. On board Jake meets Derrick's camera man Drew (Paul Scheer). The group travels to a remote part of the lake, where Drew and Derrick film Danni and Crystal swimming nude beneath the glass-bottomed boat.
Laura and Zane, bored at home, break Jake's promise and take a canoe out to go fishing. They dock at an island, and accidentally lose their canoe, stranding them both. They manage to flag down Jake on Derrick's boat, and realizing that they are the local sheriff's kids, and that he is covered in blow, Derrick fearfully allows them to board. Laura and Zane peer out through the boat's glass bottom, and see many Piranhas swimming beneath them.
Back at the beach, Julie and Fallon order an evacuation, which is promptly ignored by the partygoers. The piranhas arrive and kill many partygoers, forcing the survivors to flee for higher ground. Notable deaths include the cowardly Todd, who commandeers a motorboat, runs over and kills many injured people still in the water, until his engine stalls and his boat is capsized. Party Animal (Eli Roth) is decapitated by an oncoming boat. Fallon grabs an outboard motor, and wielding it like a chainsaw, shreds numerous Piranhas until he is overcome by them, and killed when they literally eat his feet off. Novak seizes a skidoo and a shotgun, saving as many people as he can, and the survivors make their way out of the water.
Derrick, now high on cocaine, demands that they move to a new shooting location. With Drew driving, the boat is entangled in a mass of seaweed. Angrily, Derrick takes control of the boat, accelerates, breaks free from the seaweed, and slams into some underwater rocks. The glass bottom is shattered, and with Kelly trapped below deck, water and piranhas begin to flood into the boat. Derrick, Drew, and Crystal are thrown overboard. Drew and Crystal are immediately killed as Derrick swims back to the boat. With Derrick clinging to the side of the boat, Jake and Danni manage to pull him aboard, shocked to find that most of his legs, and genitals, have been eaten by the piranhas. Derrick seemingly dies, and Danni places a beach towel over his corpse.
More and more piranhas swarm into the boat, threatening Kelly, who is still trapped below. Jake attempts to pull Kelly to safety through a skylight, which is too small for her. Jake finally calls his mother on his phone. Julie answers, and with Novak at her side, they steal a speedboat. Below deck, Kelly bats off Piranhas with a frying pan. Julie and Novak arrive, carefully reverse towards Derrick's sinking boat, and toss a tow-cable across to Jake, who ties it off to the deck. Julie, suspended over the water, shimmies across the cable to Derrick's boat. Danni, Laura, Zane, and Julie cross back over to the speedboat, as numerous piranha leap out of the water, trying to eat them. A piranha snags Danni's long hair. She falls into the water and is quickly eaten alive. Laura, Zane, and Julie make their way to the speedboat, and watch as Jake unties the tow-cable. Intending to save Kelly, he grabs Derrick's still-not-dead corpse, lobs it over the side, distracts the piranhas long enough to swim under the boat with the tow-cable. Jake opens the valves on the boat's propane tanks, lights two flares, and gives Novak the signal to step on the gas. Jake and Kelly are yanked out of the boat, just seconds before the entire boat explodes, killing all the piranha within a few hundred feet. Jake and Kelly are towed back to the speed boat, and hoisted on deck, mostly uninjured.
Happily, Julie, Novak, Jake, Kelly, Laura and Zane breathe a sigh of relief. Julie's radio suddenly crackles. Goodman is on the other end. Goodman tells her that upon further inspection, the Piranha specimen he has in his tank, and all the other piranhas in the lake, lack reproductive organs and are fundamentally babies. Novak jokes, wondering what an adult piranha would look like. Suddenly, a giant, shark-sized piranha leaps up on to the deck, eating Novak.