In Host, Missouri, the newcomer vet, Dr Eli Rudkus, is called by a farmer to look at one of his cows. The vet finds a strange parasite in the animal and sends it to the agriculture lab to be looked into. He soon finds another one in a creek and he tells the townspeople the cause might be in the animal's feed. It's soon discovered the meals are actually an experimental genetic cocktail which is causing the parasites inside the cattle and people.