Venice, at Carnival time. Silvio Steno, Italy's greatest Shakespearean actor, prepares to play Othello while his wife Simonetta (his Desdemona) is more interested in fripperies. Silvio's sister Ottavia, who is also his secretary, tells her brother that he allows Simonetta too much freedom, and that she sees too much of Count Andrea Scipione. Silvio rubbishes the suggestion, as Andrea is his best friend. "Friendship between men is no guarantee of faithfulness where a woman is concerned" says Ottavia, darkly. Silvio tells her to drop the subject.

Lelio, Simonetta's dissolute brother, approaches Silvio for a loan, but he is turned down on the grounds that he has too many debts already. Andrea asks Silvio why he's playing Othello, a tragic part, at Carnival time. Silvio replies that the role fascinates him and that Desdemona is a wonderful part for Simonetta. However, he thinks that Shakespeare was mistaken in his psychology of Othello - he thinks his jealousy is totally unconvincing.

While Silvio is rehearsing (with his small son Nino 'helping'), Andrea woos Simonetta by saying that Silvio is too bound up in his work to be a true lover and reminding her of a tender moment that passed between them previously. Unfortunately, Lelio enters at this point, and witnesses what appear to be tender scenes. After Andrea has left Lelio asks Simonetta for money. She refuses and he threatens blackmail.

When Silvio emerges, Lelio jokes about winning a bet with Simonetta. Ottavia tells Silvio that this is an excuse to conceal something else. When they're alone, Silvio asks Simonetta why she gave Lelio the money. She protests that he doesn't trust her. He says he does, but he doesn't want them to have any secrets.

Andrea sends a note (via Lelio) to Simonetta asking her to meet him that evening. Silvio receives a telegram telling him that his old master Donati is dying in Milan. Despite Simonetta's protestations, he resolves to go and see him. Simonetta pleads with him not to go - "I am afraid - afraid of myself" - but to no avail.

Andrea serenades her and climbs up a rope to her balcony. Simonetta says she cannot go to the ball as Silvio is in Milan. Andrea plies her with wine and persuades her to show him her costume, a flowery affair representing Bacchante. They leave together for the ball.

Having missed his train, Silvio returns home to find Nino in tears, saying that he's been abandoned. Silvio goes to the ball in pursuit of Simonetta, but everyone's masked and he doesn't recognise her. Simonetta returns to the apartment, to be confronted by Silvio, who starts to strangle her but stops after being horrified at his actions. She locks herself in her bedroom.

By the premiere, Silvio and Othello are indistinguishable. Scenes from the play mirror events in his own life, and the actors backstage comment on how terrifyingly convincing he is. Simonetta hands a note to Lelio, which Silvio intercepts, reading it in a terrible rage. He summons a messenger to fetch him Andrea, but has to go back on stage when he arrives. While waiting, he finds the note which says "Say I was with you last night at the Carnival ball -Simonetta".

Silvio arrives and tells Andrea that Simonetta has a lover - and wants Andrea to help him find who it is. He threatens a terrible vengeance on the man in question. Andrea tries to put Simonetta's case, but this is undermined when Simonetta reveals that she was with Andrea. Silvio is distraught, but the show must go on.

The play reaches its climax, with Silvio mixing up his lines and referring to Andrea and Simonetta instead of Shakespeare's characters. He starts strangling Simonetta for real, but this is quickly broken up. Once Simonetta has recovered, she banishes the crowds and talks to Silvio alone.

She says she thought Silvio no longer loved her and was counting on the Carnival to rekindle the flame, and was sick with disappointment when he went to Milan. Andrea accompanied her purely because he was available - his ardour left her cold. She challenges Silvio to take her home or leave her there and then. After much hesitation, Silvio chooses the first option.