Two powerful women walk on to the stage in identical clothes, black velvet trouser suits. There is a thrill of tension. One will play Mary Stuart, Mary Queen of Scots, the other will play Queen Elizabeth I. The audience doesn’t know who will play who, and neither do they. At each performance, the spin of a coin will decide. When the courtiers bow to the queen, the die is cast, the play starts.

On the first night of this truly magnificent adaptation of Friedrich Schiller's play, Mary Stuart, by director Robert Icke at Cambridge Arts Theatre, Juliet Stevenson played Mary and Lia Williams Elizabeth.

In real life, the two Queens never met, though Mary begged Elizabeth to see her, but Schiller grants Mary's wish.

The play's beautifully balanced lines give each side what seems an incontestable argument until the other counters it. The lines have a Shakespearian wit and wisdom. “Two rings” says Elizabeth, “can start a marriage or a chain.”

History regards the two (or their fates) as different sides of the same coin. Elizabeth was declared illegitimate after her father, Henry VIII, when he had her mother Anne Boleyn beheaded. This was an object lesson in how men's minds could change. No man would take Elizabeth's heart. Mary had three husbands, the first, the heir to the throne of France, when she was still a child. Her third husband was the man who had murdered her second. She was a servant of her passions. Elizabeth was revered by her people, Mary's had her locked up.

Though Mary's life lies in Elizabeth's hands, the English Queen knows that her own, too, lies at the whim of the people. “The ways things appear is the way they are”, she says. People don't look deeper, they accept what is on the surface. She laments: “To serve the people is to be a slave, the crown is a prison with jewels” and she adds: “They change their minds with every breeze that blows.”

The performances here are so powerful that at the end of the night the audience didn't want to leave the theatre. Such was the atmosphere created that no one wanted to break the spell.

Stevenson and Williams create living, flesh and blood people, human, indelible, transcendant. These are performances you see only a few times in a lifetime. There is brilliant support from Elliot Levey as the lawyer Burleigh. We all know him, reasoned, clever, feet on the ground. He's funny. We see what he is going to say in his face a split second before he says it. Carmen Munroe gave a smooth, understated, natural performance as Mary's nurse. The courtiers, Talbot (Michael Byrne) Paulet (Christopher Colquhoun), Leicester (John Light) and Mortimer (Rudi Dharmalingham) created contrasting compelling characters.

The play is in modern dress with everyone in black and white and grey, against a grim, brick set as the inside of a castle tower. The courtiers are men in suits. It works, any other costume would detract from its seriousness. Yet right at the end, after Mary's execution, Elizabeth is hidden under white make-up, a ginger wig studded with pearls and a huge farthingale dress, which the courtiers layer on top of her. The woman has disappeared under the weight of the role.

This is a three-hour show which melts like minutes. All we wanted was to see it again. Mary Stuart is at Cambridge Arts Theatre until April 28.