Young Billy can't keep Digby the lovable sheepdog he brought home from the pound, so he decides to leave him with animal expert Jeff (Jim Dale). But while Jeff's back is turned , Digby accidentally drinks a top-secret chemical that makes him grow...and grow...and grow! The gigantic dog is soon being chased all over the country. The army thinks he's dangerous and want to blow him up, and two thieves are trying to sell him to the circus! In this frantic and hilarious race against time Billy and the hapless Jeff must get to Digby with the antidote or lose him forever!