Graeme and Tim are hungry, and want their dinner — however, there is only soggy lettuce and potato peel to eat because Bill has fed their normal food (and wine) to "Bunter", a guinea pig, with dessert to follow.

The Goodies' animal "patients" include a gigantic-sized snake, a gold fish, a hen (which escapes from the basket en route to the office), a large dog, a bushbaby, a tortoise, a mongoose, a vampire bat, two singing dogs, and a tiny fluffy white kitten called "Twinkle".

The following morning, Twinkle destroys St Paul's Cathedral and the Post Office Tower, as well as squashing Michael Aspel with his huge paw, and frightening various people and dogs.

Graeme makes an antidote to counteract the disastrous effect of his growth mixture and reduce Twinkle back to the normal size of a cat, but the Goodies have to disguise themselves as giant mice, and become airborne on their trandem, to be able to get close enough to Twinkle for the antidote to be successful — following which their "hot-air trandem balloon" is carried away by a Concorde airliner.

However, the Goodies then discover that there is yet another unexpected and unforeseen consequence resulting from Graeme's growth mixture — they now have a bunch of king-size mice problem on their hands.