In a genetic research facility, a genetically altered mutant cat is placed inside a house cat. The house cat escapes captivity, and kills several people in the building before escaping through an air duct. The next day, it jumps on a truck driven by two men, and kills them both by attacking them, causing them to crash the car.
Meanwhile, Walter Graham, a multimillionaire nicknamed "Wall Street," and his associate Mike Harvey are preparing to take a luxury yacht to the Cayman Islands to evade criminal prosecution. Accompanying them are Rachel, the boat's captain who is working her way to buy the boat back from Walter; Albert, Walter's friend; and Suzanne and Bobbie, two spring breakers invited to come along. The plan is ambitious, but the promise of escape and wealth weighs heavily on everyone involved.
Before the trip starts, Suzanne and Bobbie meet three boys, Martin, Corey, and Lance, at the marina, and invite them to come along. Walter reluctantly agrees, but only under the condition that they work as the boat's crew, since the previous crew had quit. The cat somehow finds its way to Suzanne and is brought aboard, even as Walter playfully protests that the cat is not invited.
After the ship sets sail, the passengers throw a party, but trouble immediately stalks the voyage. Drunken Albert is found dead on the deck, the victim of the cat’s predatory reach, and his body sinks into the sea. Some of Albert’s blood is discovered by the others, who dismiss it as an accident, yet Martin, a curious biologist at heart, examines the blood sample with a sextant instead of a microscope and notices an abnormally high blood cell count.
Tension escalates when Walter attempts to force himself on Bobbie, and Lance moves to intervene. Mike intervenes and shoots Lance in the shoulder, but the cat, angered by the violence, bites into Lance’s Achilles tendon. Mike is thereafter gravely wounded by the cat’s bite, and Martin concludes that the cat’s bites are venomous. Rachel and Suzanne try to call for help, but Walter destroys any means of communication, pressing forward with the plan to reach the Cayman Islands.
As the days progress, Mike succumbs to horrific spasms and dies; the survivors cast his body overboard. The cat’s presence continues to menace the ship: the next day, while Lance and Bobbie engage in intimate moments, the cat bites off a portion of Lance’s fingers. Facing a grim fate, Lance chooses to end his life by leaping overboard, and Bobbie, devastated, is drawn into the same treacherous current as the sea swallows them both.
Corey hatches a desperate gambit to lure the cat into the engine room by placing food where it can be reached. When he shoots in pursuit, a steam blast erupts, killing him in a sudden, tragic accident. The cat eventually makes its way into the yacht’s food supply, contaminating the provisions and sparking a chain of panic and bad decisions.
In a bid to survive, Rachel and Martin lock away the contaminated food, while Suzanne grows suspicious that they are hoarding supplies for themselves. She defiantly consumes a few bites of bread, only to fall victim as the poison causes the veins in her neck to bulge and pop, a gruesome reminder of the peril they face.
A storm finally rolls in, and the yacht begins to sink. Walter, Rachel, and Martin escape on a lifeboat; Walter, obsessed with his fortune, tosses two briefcases of money into the raft and darts back for the third, only to be met by the cat and killed. Rachel and Martin manage to reach a safer waters, still pursued by the cat, which closes in as the lifeboat drifts toward the Cayman Islands. They dump money from one of the briefcases into a duffel bag to lure the cat away, then shove the cat overboard. The beast clings to the floating briefcase and slowly drifts off into the open sea as the two survivors press on.
Eventually they reach the Cayman Islands, living off the two remaining briefcases as their windfall, while elsewhere the cat washes ashore somewhere, bridging the film’s eerie conclusion with a final, quiet note of danger. A small, unsuspecting boy soon picks up the cat, unaware of the danger it still carries.
