Children: Nick Park p-p-p-picks up a penguin

It's a rum kind of first-night stunt that keeps the audience locked out on the pavement. But from the amiable scrum that developed outside the London premiere of A Grand Night Out on Tuesday, some of us managed a restricted view through the glass of one very small penguin being presented to Oscar-winning animator Nick Park and the Deputy Lord Mayor of London. There was not an awful lot to see, actually. It flip-flopped about the foyer carpet for a bit, posed for the posse of flashlights, was scooped up into a small box (can a baby penguin look relieved?) and finally whisked off in a taxi. This curious ceremony over, we ticket-holders surged in, only to be confronted by a pen of large, fat sheep, peeing with malevolent force onto a small square of turf.

No one seemed remotely puzzled by these scenes. For the global success of Nick Park's animated films has ensured that every man, woman and child knows who's who and what's what in the comfortably eccentric, thoroughly English world of Wallace and Gromit - a world in which a criminal penguin can disguise itself as a chicken, or a dog sit reading a newspaper, without attracting comment from humankind. So for these characters to transmute from nine-inch Plasticine models on film to live actors on stage (or even animal impostors in a foyer) is the very least surprising thing they could do.

Still, curmudgeon that I am, I had doubted this venture, if only because it does not come from W&G's creator. But it was conceived by Andrew Dawson, whose success with the stage show Thunderbirds FAB showed his alertness to the tiny details that make or break a small-screen cult. So although A Grand Night Out is a venture for Wallace and Co, it plunders themes and catch phrases from all three of Park's films, and drops in visual allusions to cinema classics - just as Park does - from beginning to end. It's a compilation of greatest hits in which everyone knows the words, but not in which order they'll come. When tongue-tied Wallace renews acquaintance with Wendolene from the wool shop, we just know their coy conversation will at some point turn to the subject of ... gulp ... cheese.

Director Martin Lloyd-Evans could easily have opted to put the actors in foam bodysuits and masks to make them look just like their Plasticine originals, but this would have blunted the characterisations. Instead he trusts to the suggestive skills of physical theatre which this first- class, mime-trained cast is well equipped to deliver. Paul Fili-piak's Wallace is a baldy with a pale blobby nose and a sleeveless pullover that you'd swear was made of knitted Plasticine. But there any photographic resemblance to his toothy prototype ends. We accept him as the Wallace we know and love because of the studiedly jerky, not-quite-human gait; the splayed sausage fingers that constantly seek something to drum on; the potato head that shoots forward and swivels from an eager turkey neck when there's something afoot; and more than all that - the voice. The voice is pure Lancastrian treacle pud - perfectly timed, done to a T, just grand.

Gromit the dog, Shaun the sheep and the dastardly Penguin don't speak, of course, so their movement (mostly on two legs) must carry even more. Perhaps children are more theatrically literate these days, but none seemed to mind or even notice that the Penguin was a girl (Angela Clerkin) wearing a tails suit and red Doc Martens. She had caught that sinister gliding walk, like a clockwork toy on wheels, that made such an effective frisson in The Wrong Trousers. On stage, this reproduced so well that every time Clerkin appeared I felt the need to check that my six-year-old's lip was not quivering.

The plot is both simple and ludicrously complex, hinging on Tom Piper's mind-boggling set. Wallace's latest Branestawmian invention is an automated travelling theatre which, at the touch of a button, will produce a full- blown adaptation of any literary work the operator chooses (this in itself is an in-joke about the endless expandability of one clever idea - just like Parks's). You just pop a book in the slot and select from the illuminated signs above the proscenium: opera/ballet/drama/ vaudeville/horror/western ... and so on. Wallace, Gromit and Shaun are joined by stage-shy Wendolene to play all the parts, and it is not hard to imagine the mayhem that breaks out once the evil Penguin gets his flippers on the zapper.

For all the things to marvel at, there are odd lacunae in the dramatic construction of A Grand Night Out. At times the plot seems to double back on itself and repeat the same gags. Nevertheless, this show is in the very best tradition of family theatre, not just because the characters appeal across the board, but because elements of the production work at widely different levels of sophistication. It's not an original like The Wrong Trousers. It couldn't be. But for a spin-off it has a lot of spin.

Peacock, WC2 (0171 314 8800), until 10 Jan.