The horny farmer McDonald has his wicked way with a goat. Life returns to normal for the family, until the goat gives birth! The mutant off spring, called Billy, is tormented by the Farmer's sons, only the daughter feels any pity for him. It isn't long before Billy, who keeps growing bigger, plans revenge..

A widowed teacher marries again, but her hopes of her daughter accepting a stepfather her own age, and her anticipation of a birthday picnic by the river are clouded by a series of murders in the district and by a fear rooted rather nearer home.