Young Billy White (Richard Beaumont) wants a pet dog, and visits the local dog home to see his favourite, an Old English Sheepdog puppy called Digby. Unfortunately, the keeper of the kennels tells him that at three o'clock a rich lady will call to collect the dog, but he takes pity on the boy and pushes the hands of the clock forward to make it look as if the lady has missed her opportunity and that Digby has been sold. So Billy gets his wish, and takes the animal back home where he lives with his widowed mother, Janine (Angela Douglas), and his grandfather. Janine is a scientist working at the top secret military research base nearby, as is her colleague Jeff Eldon (Jim Dale), and they have devised a new formula, known as Project X, which has unexpected side effects...
The British pride themselves on being a nation of animal lovers, and this silliness provides the antidote to more sentimental wildlife films such as Greyfriars Bobby or Tarka the Otter. Written by Michael Pertwee and Charles Isaacs from a book by Ted Key, it also provides an answer to those stern faced giant monster movies where a giant insect or lizard will go on the rampage, because Project X, in an cheerfully unoriginal plot development, makes things grow giant size. At first we see a cucumber that has suffered the results of this, but judging by the title, it won't be long before Digby accidentally has a taste of the formula and sure enough that is what happens, thanks to the bumbling of Jeff and his wish to grow bigger roses for an upcoming flower show.
Jeff (Dale's amiable but accident-prone persona carried over from Carry On films is ideal here) secretly steals a small amount of Project X and takes it home with him. When he gets there, he finds that Billy has left Digby on his doorstep because he's too much to handle and Janine, whom Jeff wishes to romance but doesn't have the courage, drops by in the hope he'll arrange dinner with her instead of his rival in love, Colonel Masters (Dinsdale Landen on good form). While she's there, she unwittingly feeds Digby the formula meant for the roses and the trouble begins. The storyline tends towards the chaotic in the first half, with plot piled upon plot and a wealth of hit or miss gags added in for good measure.
At times it's difficult to tell if this is a film made for children or adults as faces familar from director Joseph McGrath's other projects continually pop up. Most prominent among these is Spike Milligan, in the role of a German psychiatrist working at the research institute and offering the chance for the legendary comedian to put on a funny voice. The psychiatrist thinks that Jeff believes himself to be a dog due to all his antics around Digby - barking, crawling through the dog flap in his door, eating dog food - offering up many moments of comic misunderstandings and getting Jeff into more trouble with his superiors. Other jokes, including a bunny girl at a roadside cafe and an inappropriate slide during a demonstration, also seem aimed squarely at the grown-ups.
Through a complicated string of events, the ever growing Digby, now on the run and attempted to be hidden by Jeff, is kidnapped (dognapped?) by two acrobatic thieves (John Bluthal and Norman Rossington) and put on display at a circus. There are parallels with that most famous of giant monsters, King Kong (Digby is even named "King" at his appearance before the public) as Digby escapes, but this is a lot more light hearted, although the tone does turn needlessly sentimental by the end. There are also similarities with the Goodies contemporary Kitten Kong episode, and this film would have benefitted from scenes with the humungous hound wandering around London, but as it is, he simply tours the countryside. It's a silly film all over, but features a fair number of laugh out loud moments ("Vot is der schnag?", Jeff's auntie and her television) and is inventive enough to carry its more earnest patches and poor special effects. Music by Edwin T. Astley, including a theme song.