Venice, at Carnival time. A woman appears to be strangled, but it's a rehearsal in which the actor Silvio Steno is playing Othello with his wife Simonetta as Desdemona. Their friend Andrea calls on them, and Simonetta seems unusually keen to spruce herself up, despite Silvio assuring her that she's adorable as he is.
They discuss the play and Silvio says that the part disturbs him - and he's unconvinced by Othello's jealousy. Andrea asks about Simonetta's Carnival costume, but it's a secret. He offers to accompany her to try on some hats, and they leave in a gondola.
While Andrea attempts to woo Simonetta, Silvio has an argument with his sister Ottavia, who wants to know why he's living in a fool's paradise and is seemingly oblivious to Simonetta's flirting. She recommends stricter discipline, but Silvio says that he totally trusts her. Meanwhile, Simonetta's dissolute brother Lelio spots Andrea and Simonetta kissing.
Simonetta returns home laden with presents - clothes for her, toys for her son Nino. When Silvio takes Nino to bed, Lelio visits Simonetta and demands money on pain of telling Silvio about her dalliance. Silvio returns to see her handing over the money, which Lelio claims is a bet that she lost. When alone, Silvio quizzes Simonetta about the money, and tells her that she can't go to the ball.
Amid the noise of fireworks and general revelry, Simonetta weeps in her room. She shows her costume to Silvio, and is delighted when he changes his mind and says that they'll go to the ball - but her hopes are dashed when he receives a telegram from Milan saying that his old master Donati is dying, and rushes off to the station.
Andrea slips into their house and Simonetta's bedroom. She's shocked at first, but is won over by the promise of the Carnival - "but just for an hour". Meanwhile, Silvio has missed the last train to Milan, returns home and is horrified to find Nino distraught, terrified by the fireworks and by the empty house. While the ball continues, Silvio sits in his room, which has a view of the neon sign announcing his appearance as Othello.
At the ball, Andrea tries to kiss Simonetta and is rebuffed. He plucks a star from the breast of her costume as a souvenir. The revelry is over and Simonetta returns to find a furious Silvio demanding who she was with. When he strikes her to the ground, they are both shocked.
The first night of the play. Silvio is nowhere to be seen and the media are beginning to smell a rat. The understudy is summoned and Simonetta is panic-stricken - but Silvio finally turns up at the last minute, completely impassive and seemingly indifferent to everything. Applying his make-up, he remembers Ottavia's warning: "Sooner or later there's bound to be someone - it's only natural".
Simonetta hands a letter to her maid to give to Lelio, but Silvio intercepts it - and it reads "If you are questioned swear you were with me at the Carnival". He is about to demand an explanation when the overture ends and he is required on stage. Andrea watches the play from a box and is alarmed at the way art imitates life.
While Simonetta is singing the Willow Song onstage, Silvio summons Andrea backstage and asks him to help find Simonetta's lover. Andrea takes out his handkerchief and the star falls out. Silvio recognises it immediately and demands an explanation. Simonetta appears and explains she was at the Carnival with Andrea. Silvio is about to hit Andrea when he remembers his earlier dismissal of Othello's jealousy.
He's summoned back to the stage for the final scene, though he's so distracted he has to be reminded which it is, and he's even more distracted when on stage, substituting Simonetta's name for Desdemona's. Finally, he tries to strangle her before collapsing in a faint.
Once the commotion has subsided, Simonetta tells him how disappointed she was to miss the Carnival - going with Andrea was a moment of madness, but she was never unfaithful. They embrace, he smearing his make-up on her face. They look in the mirror and laugh.
