UNINVITED

At 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, multimillionaire "Wall Street" Walter Graham, 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 Bobbi, two spring breakers invited to come along.

Before the trip starts, Suzanne and Bobbi 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 as the other crew had previously quit. The cat finds its way to Suzanne, who also brings it along, only after Walter playfully and rather ironically protests that the cat is not invited. After the ship sets sail, the passengers have a party, although a drunken Albert is killed alone on the deck by the cat, and falls into the water. The others find some of Albert's blood the next day and dismiss his death as accidental. Martin, ever the inquisitive and enterprising biologist, nevertheless inspects the blood sample using a sextant in lieu of a microscope and observes that Albert's blood cell count was abnormally high.

Walter later attempts to rape Bobbi and Lance tries to stop him, but Mike intervenes and shoots Lance in the shoulder before the cat—incensed by the frenzy of violence—bites into his Achilles tendon. Mike is critically wounded by the bite, and Martin deduces that the cat's bites are venomous. Rachel and Suzanne attempt to call for help, although Walter destroys any means of communication, intent on reaching the Cayman Islands. Mike soon begins having horrible spasms, and eventually dies; the survivors throw his body overboard. The next day, while Lance and Bobbi are engaged in precoital foreplay, the cat bites off a couple of Lance's fingers. Resigned to his fate, Lance commits suicide by jumping off the ship. Bobbi pleads for him not to, and they both fall over and drown. Corey places several pieces of food in the engine room to lure out the cat, but when he tries to shoot it, he causes a steam blast that kills him.

The cat gets into the yacht's food and infects it, resulting in Rachel and Martin locking the food up. Suzanne believes they are hiding it for themselves and eats a few bites of bread, only for the poison to cause a few of her neck veins to pop, killing her. A storm arrives, and the yacht begins sinking, resulting in Walter, Rachel, and Martin evacuating on one of the lifeboats. Walter throws two briefcases of money into the lifeboat, and goes to retrieve the third, but is killed by the cat. Rachel and Martin escape in the lifeboat, but the cat follows and attacks them. They dump money from one of the briefcases into a duffel bag before knocking the cat overboard, watching it grab onto the briefcase, and slowly float away. The two reach the Cayman Islands sometime later, living off the two remaining briefcases of money.

Meanwhile, the cat washes up on a beach somewhere, and a little boy picks it up, unaware of the danger.

The mutant cat continues its rampage aboard the sinking yacht, resulting in the deaths of several characters, with only Rachel and Martin escaping in a lifeboat.

The film's ending emphasizes the relentless nature of the mutant threat, as the survivors are forced to abandon ship while the infected cat dispatches the remaining antagonists. It delivers on the 'no one is safe' premise typical of creature features, leaving the audience questioning whether true escape from scientific folly is ever possible.