Children's shaggy dog story. Digby is a sheepdog rescued from the pound by schoolboy Billy. When Billy is told that he cannot keep the canine at home, animal expert Jeff (Jim Dale) offers to take Digby in. However, when Digby accidentally drinks a secret chemical concoction, he grows to a massive size, going on the rampage around the country. It is up to Billy and Jeff to track the enormous Digby down before he is blown up by the army or sold to the circus!