Master Class, Vaudeville Theatre, London

"Is this a classroom or a circus?" asks Maria Callas at one point in Terrence McNally's 1995 piece that is partly based on the masterclasses that La Divina, her voice now wrecked, gave at the Juilliard School in New York in the early 1970s.

The answer inclines overwhelmingly towards the second of these options. And it is the debased, cliche-ridden version of Callas, served up here, that drags it down to that level. The character has as much in common with Dame Edna as with the great artist which this "event" or "entertainment" (you couldn't call it an even half-way decent play) travesties.

Let me say at the outset that Tyne Daly is astonishingly good at what she is asked to do for this show. Bulky in black trouser suit, features pulled back in a mask of quasi-oriental disdain, she has as much"presence" as the Grand Canyon or the Taj Mahal by moonlight. In all masterclasses, there is a tension between the public display of pedagogic purpose and the potential blood sport of pupils being made examples of in more ways than one. McNally's trite idea is to present Callas as Quintessence of Diva. It would have been more honest to have a comedy starring a generic focus-pulling monstre sacre but Callas is too useful to him on the tired "her life was as operatic offstage as on" grounds. So he makes distorting sport of her actual teaching achievement, with the pupils reduced to insulted triggers that send her floating off into melodramatic reveries about her past, staged in a La Scala of the mind. No irony is left unthumped to death as we hear of the fatal love affair with Aristotle Onassis who didn't even like opera ("A great ballerina dancing for blind man") and who forced her to have an abortion before dumping her for Jackie Kennedy.

"Forget all about me. Poof. I'm invisible," declares Daly's Callas with all the sincerity of Dame Edna singing about her "shyness". In Stephen Wadsworth's slick production, Daly times the airy, throwaway put-downs and the mock-puzzled "bitchy, moi?" double takes to perfection. And she is able, simply through charged statuesque gesture, to suggest the singing genius that we hear on scratchy recordings of Callas in some of her great roles. There is one electrifying sequence where the diva's taunts goad a young soprano into a thrilling retaliatory rendering of an aria by Verdi's Lady Macbeth and as the pair orbit each other, you see how the psychodynamic of the masterclass relationship could be tacked non-naturalistically, In general, though,you learn more about the nature of teaching and passing on a gift in Dirty Dancing.

Booking to 28 April