Dan Reilly, Dan Reilly, runs a struggling farm in rural Ireland and finds himself drawn into a secret project funded by a bio-genetics company that promises faster-growing cattle. To oversee the work, the firm sends Orla, a local veterinarian, to inspect the herd and keep the operation running smoothly. During a routine palpation, Orla appears to be bitten by the unborn calf, a clue that something unusual is happening beneath the surface. She reports the oddity to John (the firm’s genetic scientist), but he brushes off her concerns and insists the matter stay quiet. Dan, meanwhile, is told to keep the settlement quiet and to push back any locals who might threaten the secrecy surrounding the experiments. He heads out and confronts the farm’s occupants, Traveller Jamie and his girlfriend Mary, and orders them to leave by dawn. That very night, one cow goes into a difficult labor, forcing Dan to seek Jamie’s help. Together they manage to birth the calf, which bites Dan in gratitude. In a reluctant gesture of hospitality, Dan allows Jamie and Mary to remain on the farm, even as they’re on the run from Mary’s brothers who disapprove of her relationship with a Traveller.
The following day, Orla returns to inspect the newborn and is shocked to discover the calf has fangs. When Orla tries to terminate the calf, its mother lunges to defend it, and Orla is forced to kill both mother and child. A necropsy reveals the calf was somehow pregnant with six foetuses, each grotesquely deformed with skeletons protruding outside their bodies. The discovery triggers a heated clash between Dan and Orla, and they barely notice when one of the foetuses slips away. The investigation hints at a pathogen far more troubling than anything they’ve seen before, and the laboratory’s data begin to suggest extraordinary and alarming growth.
The next morning, Jamie notices several strange wounds on a cow as he passes through shallow water in the barn; something bites him as well. A police officer arrives to inform Dan that Orla is missing, and his worry grows. That night, John returns to check on the project and learns of the malformed foetuses. He discovers that their cells are multiplying at an extraordinary rate. Back in their caravan, Mary senses unusual changes in Jamie’s behavior as they are together; they’re awakened by a creature crawling in their bed, which quickly escapes. John orders a full quarantine of the farm, warning that the creature could spread to other cattle and trigger a wider outbreak.
A search ensues for the mysterious being, which hides within the murky water that floods the barn. Dan tries to flush it out with a tractor but instead discovers Orla’s body floating in the water. John transports her remains to the lab, where he finds foreign material inside a wound in her abdomen. The material mirrors the earlier rapid cellular growth and now contains human cells, revealing that humans can be vulnerable to the infection as well. Realizing the severity of the threat, Jamie tries to escape after learning of his own infection, but John kills him to prevent further spread of the disease. When Dan and Mary come outside, they discover that John has killed almost every cow on the farm in a bid to locate the creature. He explains that the parasite has been hiding inside the cows, feeding on them from within and growing larger with each passing day.
They hurry to the barn where the final cow is kept, only to find that the calf has already been born and has escaped. A violent confrontation erupts between John and Dan; John is dragged into the melee and is killed by the creature, which pursues Mary as she flees. Dan barely survives and helps Mary by creating a distraction, using noise to lure the creature away. He sacrifices himself to give Mary a chance to strike, splashing the creature with boiling water before finishing it off. With the danger temporarily contained, Mary flees the farm, carrying the weight of what she has witnessed.
Four months later, in Birmingham, England, a pregnant Mary receives reassurance from her doctor that her baby appears healthy. Yet the ultrasound monitor reveals a brief glimpse of a creature-like shape inside the fetus, suggesting that the threat may not be over and leaving the door open for a troubling possibility about what lies ahead.
