Cassio has been elected as general Othello's new lieutenant, greatly upsetting Iago, Othello's advisor. Iago reveals this to Roderigo as they arrive at the villa of Brabantio, a Venetian senator. They awaken Brabantio with the startling news that his daughter Desdemona has been secretly married to Othello, a Moor. As Brabantio rushes to Othello's home, Othello receives a summons from the Duke to a council of war. A fleet of Turkish ships has been sighted—an augury of war—and Othello is dispatched to Cyprus, followed by Desdemona. While they prepare to depart for Cyprus, Iago promises Roderigo that he will help him gain Desdemona's affection. Iago secretly plots to insinuate an affair between Desdemona and Cassio.

Othello reaches the harbor after the rest of his party, with the good tidings that the Turkish fleet is scattered by a tempest. Iago convinces Roderigo that Desdemona is in love with Cassio. Iago instructs Roderigo to start a fight with Cassio; at that evening's celebration, Iago gets Cassio drunk, and a brawl breaks out between Roderigo and Cassio. Cassio injures Montano, the governor of Cyprus, in the fight. Awakening, Othello learns what has happened and strips Cassio of his office. Iago encourages Cassio to beseech Desdemona to speak in his favor to Othello.

Cassio visits Desdemona and begs her to go to Othello on his behalf. She consents, but as Othello draws near Cassio hastily takes his leave. Iago reveals his suspicion to Othello that Cassio and Desdemona are lovers. Desdemona drops a handkerchief given to her by Othello. Iago's wife Emilia presents it to her husband, who secretly stows it in Cassio's house. Having been told by Iago that Cassio possesses the handkerchief, Othello commands Desdemona to show it to him; reluctant to admit it is lost, she refuses to do so. Cassio, who has found the handkerchief, presents it to his lover, a courtesan named Bianca.

When Cassio approaches, Othello hides himself and Iago questions Cassio about his love. Cassio speaks of Bianca, but Othello believes he is speaking of Desdemona. When he sees Cassio with the handkerchief, Othello announces his plan to kill Desdemona that night; Iago agrees to kill Cassio. Iago reassures Roderigo he will win Desdemona's love, but first they must detain her from leaving Cyprus—by killing Cassio.

That night, Roderigo attacks him, but Cassio wounds Roderigo. Iago wounds Cassio, and murders Roderigo. Othello goes to his chamber and suffocates Desdemona. As she dies, Emilia enters with others and reveals the truth about the handkerchief. Iago kills her. Finally aware of the deception, Othello wounds Iago and then kills himself.