Fisherman Matthew Boyd is fishing in Lake Victoria, AZ when a small earthquake hits, splitting the lake floor and causing a whirlpool. Boyd falls in and is ripped apart by a school of piranhas that emerge from the chasm and ascend the vortex.
Jake  is admiring attractive tourists as Spring Break begins. He reunites with his old crush, Kelly and meets Derrick Jones an eccentric pornographer, as well as Danni, one of his actresses. Derrick convinces Jake to show him good spots on the beach for filming a pornographic movie.
That night, Jake's mother, Sheriff Julie Forester , searches for the missing Matthew Boyd with Deputy Fallon , who has found Boyd's boat. They find his mutilated body and contemplate closing the beach down. The next morning, a lone cliff diver is attacked and consumed by the marauding fish.
Jake bribes his sister and brother, Laura and Zane, to stay home alone so that he can show Derrick around the beach. After Jake leaves, Zane drafts Laura to go fishing on a small sandbar island. They forget to tie the boat down and are stranded in the middle of the lake. Meanwhile, Jake goes to meet with Derrick and runs into Kelly, who invites herself onto Derrick’s boat, The Barracuda. Jake meets Crystal , another of Derrick’s actresses, and cameraman Drew. They film an underwater love scene between Danni and Crystal before partying.
Julie takes a team of seismologist divers — Novak , Sam, and Paula  — to the fissure. Novak speculates that the rift leads to a buried prehistoric lake. Paula and Sam scuba dive to the bottom, discovering a large cavern filled with large egg stalks. Both are killed before they can escape or alert the others. Novak and Julie pull Paula's corpse into the boat, capturing a lone piranha, which they take to Mr. Goodman , a marine biologist. He explains that the piranhas are a prehistoric species, long believed-extinct, and must had been trapped underground for over two million years.
Julie, Novak, and Fallon try to evacuate the beach, but their warnings are ignored until the piranhas attack the tourists. Novak boards a jet-ski with a shotgun to help while Fallon ushers people to shore and Julie tries to get swimmers into the police boat. A floating stage set up in the water collapses from the weight of all the panicking guests and the wet T-shirt contest host is killed.
Jake spots Laura and Zane on the island, and forces Derrick to rescue them. Derrick gets the boat's propeller stuck and crashes into some rocks, flooding the rooms below deck. Kelly is trapped in in the kitchen while Derrick, Crystal, and Drew are thrown from the boat. Crystal and Drew are devoured while Danni manages to get a partially eaten Derrick back on board. Jake calls Julie for help.
Julie and Novak commandeer a boat and take it to the sinking Barracuda. Fallon stays behind to fight off the piranhas with a shotgun in shallow water. Seizing the motor off a speedboat, he turns it on and shreds much of the swarming school with it, sacrificing himself but giving more swimmers a chance to get out of the water.
Julie and Novak reach Jake and attach a rope to his boat. Julie, Danni, Laura, and Zane start crossing the rope but the piranhas latch onto Danni's hair and pull her into the water, devouring her. Laura, Zane, and Julie make it to safety but Jake breaks the rope. Using Derrick's corpse as both distraction and bait, Jake ties the line to himself and goes to save Kelly. He ties Kelly to him and lights a flare after releasing the gas in a pair of stored propane tanks. Novak manages to start the boat after a struggle and speeds away just as the piranhas surround Kelly and Jake. They are dragged to safety and the propane tanks explode, destroying the boat and killing most of the piranhas.
Mr. Goodman calls Julie on the radio, telling her that the glands on the piranha they obtained were not mature, meaning that the piranhas they've been fighting are babies. Novak wonders where the parents are. A giant adult piranha then lunges out of the water, attacking Novak.