The Shadow of a Boy, Ny Lyttelton Loft, London

Visitors from another planet

"The planet Earth," the 11-year-old Luke intones at the start of The Shadow of a Boy, "is not fit to join the Galactic League of Civilisations." Characteristically, Luke's comic-book fantasy ends with his deciding to remain on our benighted planet and use the knowledge of his superior universe to bring us up to snuff.

Luke is a quiet, thoughtful, helpful boy, obedient to the God-fearing grandmother who has cared for him since the death of his parents in a car accident. But another influence besides Nanna and comics enters his life – Katie Fletcher, an older girl, who taunts him with her knowledge of sex and swearing and playground power tactics. But Luke bides his time and finally gets his own back, frightening Katie with some gruesome details of how she and her mother will be melted down by an imminent atomic blast.

Gary Owen's play is one of those irritating works about children that is not written for them, but doesn't have much to offer adults, either. The tone is as mild as podgy, plodding Luke, and the action tame: Katie teases Luke, who tries not to cry and hangs his head; Nanna makes tea for Luke and tucks him into bed.

The only other character is Shadow, who rises from Luke's comic books to become his (sorry if the next two words make you retch) imaginary playmate. He allows Luke, for once, to be in charge, as Shadow needs every facet of life on Earth explained to him – that flowers, for instance, are not made of their homophone, and that foxgloves are not cold-weather wear for little paws. (Actually, this part does seem to have been written for children.) But it doesn't take him long to grasp the main problem with earthlings: "Back where I come from, we're exploring the galaxy and figuring out the secrets of the universe and unearthing the ruins of ancient civilisations, and down here you're all rushing around and trying to scrape together the cash for a Betamax."

Nanna, too, thinks that a lot of earth-dwellers don't have their priorities right: "I tell her: a woman like yourself who can't even make a bara brith for the harvest festival – and still you think you know better than the God that made the sun and the sky and the stars, and you won't send little Katie to Sunday school." This speech tells us that the play is set in Wales, but thereafter we must keep telling ourselves, because, for all the social detail, it might be in some parallel galaxy.

The actors who play the children are much older than their roles, and do not convincingly act younger. The two notable features of Erica Whyman's production are Soutra Gilmour's cruciform set, where flowers and weeds spring from cracks between the floorboards, and Jo Stone-Fewings's appearance, in a green-and-orange leather catsuit as the visitor from the planet that, for all its virtues, doesn't seem to have a good outdoor-sports shop.

To 29 June (020-7452 3000)