In the second act Jago proceeds to carry out his evil intents, by sending Cassio to Desdemona, who is to intercede for him with Othello. Jago then calls Othello's attention to the retiring Cassio and by making vile insinuations inflames his deadly jealousy. Desdemona appears, surrounded by women and children, who offer her flowers and presents. She comes forward to plead for Cassio, and Othello suspiciously refuses.—She takes out her handkerchief to cool her husband's aching forehead with it, but he throws it down and Emilia, Jago's wife, picks it up. Jago wrenches it from her and hides it.
In the next scene Jago's villainous insinuations work upon Othello, who becomes wildly suspicious. Jago relates a dream of Cassio's, in which he reveals his love for Desdemona, then he hints that he has seen Othello's first love-token, her lace-handkerchief in Cassio's hands, and both swear to avenge Desdemona's infidelity.
In the third act Othello pretending to have a head-ache, asks for Desdemona's lace-handkerchief. She has lost it, she tells him, but he is incredulous and charges her with infidelity. All her protests are useless, and at length he forces her to retire. Meanwhile Jago has brought Cassio and urges Othello to hide himself. Cassio has a lady-love named Bianca, and of her they speak, but Jago dexterously turns the dialogue so as to make Othello believe that they are speaking of his wife. His jealousy reaches its climax, when Cassio draws forth Desdemona's handkerchief which Jago has deposited in Cassio's house. All his doubts now seem to be confirmed. A cannon-shot announcing the arrival of a galley interrupts the conversation and Cassio quickly leaves.
In the following scene Jago advises Othello to strangle his wife. Othello consents and gives Jago a captaincy.
Lodovico, an Ambassador of Venice, arrives with other nobles to greet their liberator Othello. Desdemona once more asks pardon for Cassio, but is roughly rebuked by her husband. The latter reads the order, which has been brought to him, and tells Cassio, that he is to be General in his stead by will of the Doge of Venice, but while Cassio is confounded by this sudden change of fortune, Jago secretly vows his death, instigating his rival Rodrigo to kill him. At last Othello faints, overcome by conflicting emotions.
In the fourth act Desdemona filled with sad forebodings takes a touching farewell of Emilia. When she has ended her fervent prayer (one of the most beautiful things in the opera), she falls into a peaceful slumber. Othello awakes her with a kiss, and tells her immediately thereafter that she must die. She protests her innocence, but in vain, for Othello telling her that Cassio can speak no more, smothers her. Hardly has he completed his ghastly work than Emilia comes up, announcing that Rodrigo has been killed by Cassio. Desdemona with her dying breath once more asserts her innocence, while Emilia loudly screams for help. When the others appear, Emilia discovers her husband's villany. Jago flies, and Othello stabs himself at the feet of his innocent spouse.