Desdemona’s interview with Cassio is cut short by the arrival of Othello. Cassio leaves hastily in order to avoid speaking with Othello. Desdemona pleads to Othello on Cassio’s behalf. When she exits, Iago says that Cassio’s avoidance of Othello is suspicious and that Cassio may not be honorable, all the while insinuating that he, Iago, knows more than he is willing to say. He warns Othello against becoming jealous of Desdemona.
When Desdemona enters and Othello complains of an aching head, Desdemona offers to bind his head with her handkerchief. As they exit, the handkerchief drops unnoticed by either of them. Emilia picks it up and gives it to Iago, who has often asked for it. Othello reenters and, now tormented by jealousy, threatens Iago with death unless he provides proof of Desdemona’s infidelity.
Iago alleges that Cassio one night talked in his sleep about making love to Desdemona and that Cassio once wiped his beard with the lost handkerchief. Othello is convinced by this “proof” and vows to kill Desdemona; Iago agrees to kill Cassio. Othello then appoints Iago to the lieutenancy.