Synopsis:A young couple is out on a boat in the middle of a cooling pond of a decommissioned nuclear power plant in northern Florida, when they are stalked and killed by a mysterious creature. The next day, LADY WINDSOR, the sheriff, and her deputy, JEREMY DUNDERLAY, head out to investigate the disappearance. When a team of paramilitary operatives led by JOHNSON arrives at the scene, Lady and Jeremy take the small remains (a severed arm, a half-chewed hand, an oar with a giant bite-mark taken out) left for examination. Lady instructs Jeremy to head to the morgue with the evidence while she heads to a pre-arranged dinner with her longtime boyfriend and current small-town mayor, BRUCE CAMPBELL. There, Bruce Campbell plans to finally pop the question to Lady, only to be interrupted by a phonecall. MOLLY, the coroner, has discovered something strange and that prompts Lady to abandon her dinner with Bruce Campbell and head over to the morgue. Bruce Campbell tags along when he realizes that he has no ride home.
Lady's dimwitted brother, MANNY, is heading out to dig holes with his trailer-park hottie when she suddenly vanishes off the path. Manny ventures deep into the swamp to find her. He is stalked by something in the water that drives him up the trunk of a swamp tree where he clings to his flask, terrified.
Meanwhile, a family of overstuffed, moon-faced, local folk have called the local police to help them get a creature out of their kitchen. Unsure of what they're dealing with, and unable to reach Lady via radio or cell, the young cops head into the home to clear the animal out. But all hell breaks loose when the beast in their kitchen devours HANK, the patriarch of the family, and chases policemen ERIK and GABE around the house, only to get tangled up in an errant garden hose. Erik uses the opportunity to decapitate the monster with an axe. However, a bigger beast emerges from the swamp and chases Erik, Gabe, and PATTI and her kids away from the scene.
At the same time, EVA, the quirky and sassy receptionist at the sheriff's station finds herself under siege by two bloated, carnivorous, LAND-WALKING, man-eating manatees. She heads for the station's armory and fights them head-on.
As Lady arrives at the morgue, Molly informs her that an animal must have been responsible. She tells Lady that she's called in an expert to assist her. With impeccable sense of timing, marine biologist and Lady's ex-boyfriend TOMMY RIVERS drunkenly bursts into the scene. Immediately, he badgers Lady about their past relationship before she convinces him to focus on helping her solve the mystery of the severed arm. Just then, Erik, Gabe and Patti enter with her kids and the severed head of the manatee in tow. Tommy theorizes that the manatees have mutated and are now walking on land and eating people. After learning that Gabe and Erik have tangled with these monsters, Lady tries to call Eva. Leaving the morgue she is immediately confronted by Johnson and his men who reveal their intentions to cover up the manatees, and that Bruce Campbell played a part in the conspiracy. Devastated to learn of Bruce Campbell's lies, Lady has little time to act before all hell breaks loose as a pack of manatees descend on the parking lot, killing Johnson and his men. Everyone else splits up. Jeremy, Molly, Erik, Gabe, Patti and her kids head into town to rescue any survivors, and Tommy and Lady peel off to the station to rescue Eva. Bruce Campbell takes off into the swamp alone, leaving behind Lady and his left shoe.
Manny finally summons up the courage to climb down from his perch and stumbles into a swamp-side party of local teens. There, they are all attacked by the manatees. Manny manages to get to safety, but witnesses a gigantic manatee emerge from the swamp and mate with all the others. Disgusted, he runs away into the woods.
Jeremy's gang arrives in the center of the deserted small town. Fortunately, they find a group of survivors at Zeke's, the local restaurant owned by ZEKE, the biggest, baddest biker there has ever been. They try to convince Zeke to escape, but are soon attacked by manatees again. Gabe is instantly killed, Patti is trapped in the cruiser with her kids, and the manatees surround the joint, eventually breaking in through the barricade. Molly and Jeremy get to the roof as Zeke and Erik sacrifice themselves. A fire starts and flames seem to spell the end for Molly and Jeremy. Just then, Eva arrives on the scene, recklessly driving a firetruck all by herself. She plows into the pack of manatees, fires shotguns, tosses grenades and rescues the remaining survivors.
Meanwhile, Lady and Tommy arrive at the destroyed sheriff's station. Finding a couple manatee corpses, Tommy dives in and examines them. Manny suddenly appears, and knocks himself out, tripping on a rock. He awakens and tells Lady and Tommy that he witnessed a manatee mating ritual. Tommy discovers that the manatees are rapidly aging and he theorizes that they must mate constantly in order to survive. He believes that there is one female producing all the offspring (which are all males), and that they must kill her before she gives birth to a girl. So they load up an arsenal and head into the swamp. There, they find the mama manatee and blow her up with a crudely-constructed pipe bomb.
Trudging along the road, bloodied, bruised and looking like hell, they are picked up by Eva in the fire truck and drive off into the sunset, happy and safe.
Bruce Campbell, however, is stumbling through the woods and swamp by himself. Searching for a cell signal, he reaches the top of a small hill. But he sees a terrible sight. Dozens -hundreds- of manatees mating before him. Horrified, he tries to escape...