In Venice, Roderigo complains to Iago that, despite the money he's given Iago to help him woo Desdemona, she's eloped with the Moorish general Othello. Iago responds that he too hates Othello, for whom he works as a standard-bearer: Othello chose Cassio, rather than Iago, to be his lieutenant. The two men go to the home of Desdemona's father, the senator Brabantio, and rouse him with graphic descriptions of his daughter having sex with the Moor. Brabantio, enraged, interrupts Othello as he receives an urgent message from the Duke of Venice, and accompanies Othello see the Duke. In front of the Duke, Brabantio accuses Othello of having used magic to seduce Desdemona. Othello responds that it was stories of his exciting life history and military bravery that won Desdemona. When summoned, Desdemona supports Othello's story. Brabantio grudgingly blesses the newlyweds. The Duke then sends Othello to lead a fleet of Venetians to defend Cyprus from a Turkish attack. Desdemona accompanies him. Iago reassures Roderigo that he will still win Desdemona in the end, then privately admits that he's just using Roderigo for money while he plots his own revenge.
When the Venetians arrive in Cyprus, the governor Montano reports that a storm at sea has drowned the Turkish fleet, eliminating the military threat. Iago quickly hatches a plan to make Othello believe that Desdemona has been unfaithful to him with either Cassio or Roderigo. That night, while Othello and Desdemona go to bed to consummate their marriage, Iago succeeds in getting Cassio drunk. He then goads Roderigo into provoking Cassio, starting a brawl. Disgusted, Othello demotes Cassio.
Meanwhile, Iago convinces Desdemona to try to get Othello to reinstate Cassio. Iago then uses Desdemona's requests that he be merciful to Cassio to make Othello suspect that Desdemona is cheating on him with Cassio. Othello, takes the bait, repeatedly praising Iago for his honesty. Later, when Desdemona accidentally drops a handkerchief that Othello had given to her as a love-token, Emilia gives it to Iago, who had long asked her to steal it for him. Iago then plants it in Cassio's room.
Othello, upset, demands that Iago show him proof of Desdemona's infidelity. Iago responds that he has heard Cassio fantasize lewdly about Desdemona in his sleep and that he has seen Cassio wipe his mouth with Desdemona's handkerchief. Othello promotes Iago to the status of lieutenant and orders him to kill Cassio within three days. Othello then goes to Desdemona's room, and asks her for the handkerchief. Desdemona, who had been searching for the handkerchief, admits she can't find it. Othello storms off. Meanwhile, Cassio's mistress, the prostitute Bianca, comes to his quarters. Cassio asks her to make a copy of a handkerchief he's recently found in his room, because he admires it.
Iago continues to spur Othello's jealousy. When he reports that Cassio has admitted to sleeping with Desdemona, Othello falls into an epileptic fit. Iago urges Othello to hide while he questions Cassio about Desdemona. In fact, he asks Cassio about Bianca, causing Cassio to laugh. Watching from afar, Othello grows increasingly furious. Then, Bianca shows up, and throws Desdemona's handkerchief at Cassio, accusing him of having it from another whore. After Cassio and Bianca leave, Iago easily persuades Othello to kill Desdemona. Iago promises to take care of Cassio himself. He then convinces Roderigo that if Cassio were to die, Othello would have to remain in Cyprus, leaving Desdemona in Venice for Roderigo. Iago instructs Roderigo to wait outside Bianca's house that night and kill Cassio when he leaves.
That night, Iago sets Roderigo up to kill Cassio as planned. When Cassio exits Bianca's house, Roderigo attacks him; both are wounded. Overhearing Roderigo's cries for help, Othello believes that Cassio is dead and is impressed by Iago's loyalty. Meanwhile, Iago goes to Bianca's; finding Cassio wounded, he stabs Roderigo, killing him (and thus assuring that his secret will not be revealed). Iago then calls the others, including Bianca, whom he arrests, accusing her of having conspired with Roderigo. While this is going on, Othello arrives at Desdemona's chamber. Enchanted by her beauty, he nonetheless resists her pleas to spare her life, and he smothers her with a pillow. Emilia arrives to tell Othello that Roderigo is dead and Cassio alive, when she hears Desdemona's dying cries.
When Emilia demands why Othello has killed Desdemona, Othello explains how Iago proved to him that Desdemona slept with Cassio. As Montana, Iago, and Gratiano, a relative of Brabantio's all arrive, Emilia accuses Iago of lying and explains that she stole this from Desdemona at her husband's behest. Othello attacks Iago. In the uproar, Iago stabs and kills Emilia, then flees. Montano and Gratiano disarm Othello, then chase down Iago. When he is dragged back in their custody, Othello wounds him before being disarmed again. Letters found on Roderigo's corpse reveal the full extent of Iago's plots; he himself refuses to explain himself. Othello draws a hidden dagger and, after a speech, kills himself.