To many people, the name Maria Callas conjures up the best and worst about opera singers. On the one hand, there was her extraordinary voice, with its wide range, immense emotional power, and ability to communicate. On the other hand, there was the diva-like behaviour: stories abound of rows and tantrums.

Terrence McNally’s play Master Class includes both sides of the picture. It’s set in the Julliard School, New York, where Callas, in real life, gave master classes in 1971-2. Three aspiring singers are lined up for her attention. First on is Sophie de Palma (Robyn North), who arrives late. “I was never late,” snaps Callas, “I lived, breathed, and worked music.” Callas interrupts her student’s singing after only a couple of bars in order to berate a stagehand (Scott Hazell), who has failed to produce a demanded cushion and footstool. Only the long-suffering accompanist (David Harvey) escapes a sarcastic tongue-lashing, even though Callas affects not to remember his name from one minute to the next.

“This isn’t about me, it’s about the students,” Callas says, in one of many telling asides to us, the theatre audience. Of course, it is all about her. She adds: “Sooner or later you will catch on to my sense of humour — or you won’t”. We are told that she never runs down her former operatic colleagues, but this resolve lasts only a few seconds. “Joan Sutherland did her best” is one of the kinder comments.

Sophie is followed by Sharon (Pamela Hay), who wants to sing Lady Macbeth. “Then enter like Lady Macbeth,” she is told, but Callas isn’t finished yet: “And never wear a dress like that before midnight at the earliest”. Sharon flees without singing a note, to throw up in the loo. But she returns, to great acclaim: “You have shown courage, what the Germans call Mut”. Final student Tony (Christopher Jacobsen) reduces Callas to momentary silence when he smugly announces that he wants to sing like Mario Lanza.

Master Class is expertly written, with some of the most moving scenes featuring Callas reminiscing about her great days, against a ghostly backdrop of La Scala, Milan, and with her real singing voice in the distance. But everything depends on the actor playing Callas, and this Theatre Royal, Bath, production (director Jonathan Church) has a winner: Stephanie Beacham’s performance in the lead role is quite simply mesmerising. Whether it’s giving advice to the students, or telling us, with both bite and melancholy, about her affair with shipping magnate and fellow-Greek Aristotle Onassis (“He told me: ‘Before you were a canary, now you are a freak who f***** Aristotle Onassis’”), not a word seems wrongly delivered or out of place. Beacham must surely win a major award for this performance.

Master Class continues until Saturday. Tickets: 01865 305305 or online (ww.oxfordplayhouse.com).