Before his adventure begins, Digby is given to Bill by a friendly vet, but as Bill's grandfather doesn't like to have an English sheepdog in his house, Bill is forced to give him away. He gives Digby to a guy who works in a defense lab that experiments with animals and plants for space travel. A growth-boosting chemical is made in the lab, and the animal trainer steals some to boost his own tomato crop. Accidentally, the dog is fed some and, as a result, is growing all the time; eventually, he's the size of an ocean liner. As weird animals are attractions, Digby is stolen by two well-dressed hoods and sold to a circus, from which Digby escapes, hunted by the military, both Army and Air Force!