Bat Boy: The Musical, Shaftesbury Theatre, London

A batty musical with bite

Bat Boy was first revealed to the world in a 1992 edition of the US tabloid Weekly World News, which reported the discovery, in a cave in West Virginia, of a mysterious "bat child" - two feet tall, with fangs, giant ears and an uncanny ability to see in the dark. Since then, Bat Boy has become something of a mascot for the Weekly World News, which regularly features photographs of the creature, along with accounts of his remarkable exploits: his night vision was, apparently, a decisive factor in the capture of Saddam Hussein, and earlier this year the paper reported that, in recognition of his heroic role in saving a British army patrol in Iraq from ambush, he is to be knighted - Sir Bat Boy.

Bat Boy was first revealed to the world in a 1992 edition of the US tabloid Weekly World News, which reported the discovery, in a cave in West Virginia, of a mysterious "bat child" - two feet tall, with fangs, giant ears and an uncanny ability to see in the dark. Since then, Bat Boy has become something of a mascot for the Weekly World News, which regularly features photographs of the creature, along with accounts of his remarkable exploits: his night vision was, apparently, a decisive factor in the capture of Saddam Hussein, and earlier this year the paper reported that, in recognition of his heroic role in saving a British army patrol in Iraq from ambush, he is to be knighted - Sir Bat Boy.

This is a promising basis for a camped-up, ironic entertainment along the lines of Jerry Springer: the Opera - another musical derived from the more sensational end of American pop culture. And this production, arriving in the West End after a run at the West Yorkshire Playhouse, gets off to a good start, with the chorus deadpanning couplets such as: "In a cave many miles to the south/ Lives a boy born with fangs in his mouth."

In this story - written by Keythe Farley and Brian Fleming - Bat Boy is discovered by a trio of teenagers messing around in a cave, one of whom, in his panic, he bites. Back home in Hope Falls, West Virginia, the grotesque animal is handed into the care of the local vet, Dr Parker, on the assumption that the vet will put him down. He is, however, saved by Dr Parker's wife, Meredith, who takes him under her wing and inducts him into the ways of civilisation. The show's best number has Bat Boy progressing at breakneck speed from halting, garbled syllables to a patter-song about the joys of knowledge - he can tap dance, identify a photograph of Elvis Costello, and discourse on Copernicus and Freud: "Say 'Howdy'/ To a summa cum laude." Soon, he is speaking BBC English, cocking his little finger as he drinks his tea and dressing in a white three-piece suit - Tom Wolfe meets Nosferatu. But still the townsfolk refuse to accept him, blaming him for the "plague" that leaves their cattle emaciated; and, meanwhile, Dr Parker grows dangerously jealous of the affection that his wife and their daughter, Shelley, lavish on the boy.

Good jokes and enjoyable musical pastiche (music and lyrics by Laurence O'Keefe) continue to crop up from time to time, but there are also a good many moments when the humour feels either misdirected or simply aimless, the tunes derivative rather than parodic. By the interval it is clear that the show has shot its bolt. The second half feels messy and occasionally desperate - a low point is the arrival on stage of the great god Pan, singing a hymn to the joys of natural love to the accompaniment of a chorus of rutting animals.

Mark Wing-Davey's production has some inventive ideas and good leading performances, especially from Deven May, who created the role of Bat Boy in the original off-Broadway production, and Rebecca Vere, primly sexy as Mrs Parker. The show's minor roles are, however, mostly undercast.

But the underlying problem is the writers' uncertainty about how to pitch the irony. The critic Claude Rawson has talked about irony in terms of achieving a balance between meaning it, not meaning it and not not meaning it: too often in Bat Boy, it feels as if they have settled for simply not meaning it (as when the bigoted townsfolk congratulate: "We can't rid ourselves of our Christian charity") or, worse, actually meaning it: the closing hymn to the virtues of tolerance, with its chorus about getting in touch with your inner Bat Boy, feels disturbingly close to sincerity. Like its vampiric hero, Bat Boy sometimes bites, but it also sucks.

Booking to 30 October (020-7379 5399)