In Host, Missouri, the newcomer Dr. of Veterinary Magic Eli Rudkus is called by the farmer Jacob Long to exam one of his cows. The veterinarian finds a strange parasite in the animal and sends it to a friend in the Department of Agriculture for research. Later, he finds the same parasite in a creek and he summons the population for a meeting, warning that the cause might be the animal food. However, Fletcher Odermatt, the wealthy owner of the local Host Tender Meals that has been providing free animal food for the farmers, brings his lawyer Hayley Anderson and discredits Eli. When a huge mutant parasite attacks Eli and Jacob, they discover that the meals are actually an experimental genetic cocktail that is growing parasites inside the cattle and people.