125th Street, Shaftesbury Theatre, London

A great musical could be written about the heyday of Harlem's Apollo Theatre. Unfortunately, this isn't it, writes Rhoda Koenig

The street name-checked in the title of this musical is the main artery of Harlem and the site of the Apollo Theatre, where, from 1934 to 1975, black audiences could see black singing stars. The carotid artery is the one that supplies blood to the brain, and throughout this show I kept fearing it would close down, wishing to put me out of my misery.

It doesn't take much brain activity to suppose the worst of a show with an awful premise, and 125th Street is certainly one of these. In the story cooked up by Rob Bettinson and Alan Janes (the former also directs, the latter produces), the year is 1969, and Tony Sorrento, a white variety-show host, hopes to save his dying career with an all-star special at the Apollo. But this is the first time I have had my suspicions about a show confirmed in the lobby. A television monitor carries the action on stage, where, well before starting time, Sorrento's secretary stalks back and forth, haranguing patrons as they enter. "Come on!'' she shrieks. "Get in your seats! Stop chatting! You're not in the laundromat!'' I've seen shows that try too hard to ingratiate themselves, but never one that actively puts people off coming in.

On the set, which seems to have been made out of a few chairs and a roll of tinfoil, there is yet more rancour, for Tony is taking out his anger on everyone within spitting distance. A race riot outside has prevented James Brown, Dionne Warwick and Aretha Franklin from getting to the theatre, and Tony abuses the stage crew as well as the lower-ranking acts.

Singers impersonate Joe Tex, an imitation on the accuracy of which I can't comment, and Screamin' Jay Hawkins, on which I can. Instead of an artist so intense he would send women into screaming fits of their own, we see a goofy guy who becomes an object of ridicule when his act goes wrong. Later, a black woman fleeing his supposedly bizarre sexual attentions rushes on with her dress pulled down and her knickers flapping around one leg.

The pair exemplify the show's demeaning treatment of black people – Tony's insults are close to outright racism, and his flattering encouragement of the genuine amateurs (who have been recruited from auditions around the UK) makes an unpalatable contrast with his contempt for the black actors. All three amateurs, you see, are white – this show's verisimilitude is on a par with its taste.

Since the stars never arrive, Tony has to beg the small-timers he scorned to take the stage. And do you know what happens? Of course you do: they are wonderful. Or, rather, that's the idea. The nasty secretary orders us to applaud, the joke, presumably, being that we would anyway. But the renditions of such pulverising songs as "I Feel Good'' and "Tell Him'' are only adequate – as George Kaufman said, another word for inadequate. You never feel, as you should during a succession of these songs, that the performer might drop dead, or that you will. At one point a singer lectures the Apollo's Jewish owner: "This isn't a theatre – it's a church. And today you pissed all over the altar cloth.'' For Bettinson and Janes to write such a line is like the pisspot calling the kettle black.

'125th Street', Shaftesbury Theatre, London WC2 (0870 906 3798)