Othello leaves Cassio on guard during the revels, reminding him to practice self-restraint during the celebration. Othello and Desdemona leave to consummate their marriage. Once Othello is gone, Iago enters and joins Cassio on guard. He tells Cassio that he suspects Desdemona to be a temptress, but Cassio maintains that she is modest. Then, despite Cassio’s protestations, Iago persuades Cassio to take a drink and to invite some revelers to join them.
Once Cassio leaves to fetch the revelers, Iago tells the audience his plan: Roderigo and three other Cypriots, all of whom are drunk, will join Iago and Cassio on guard duty. Amidst all the drunkards, Iago will lead Cassio into committing an action that will disgrace him. Cassio returns, already drinking, with Montano and his attendants. It is not long before he becomes intoxicated and wanders offstage, assuring his friends that he isn’t drunk. Once Cassio leaves, Iago tells Montano that while Cassio is a wonderful soldier, he fears that Cassio may have too much responsibility for someone with such a serious drinking problem. Roderigo enters, and Iago points him in Cassio’s direction. As Montano continues to suggest that something be said to Othello of Cassio’s drinking problem, Cassio chases Roderigo across the stage, threatening to beat him. Montano steps in to prevent the fight and is attacked by Cassio. Iago orders Roderigo to leave and “cry a mutiny” (II.iii.140). As Montano and others attempt to hold Cassio down, Cassio stabs Montano. An alarm bell is rung, and Othello arrives with armed attendants.
Immediately taking control of the situation, Othello demands to know what happened, but both Iago and Cassio claim to have forgotten how the struggle began. Montano insists that he is in too much pain to speak and insists that Iago tell the story. At first Iago feigns reluctance to incriminate Cassio, emphasizing the fact that he was chasing after Roderigo (to whom Iago does not refer by name) when the fight between Cassio and Montano began, and suggesting that the unknown man must have done something to upset Cassio. Othello falls into Iago’s trap, stating that he can tell that Iago softened the story out of honest affection for Cassio. Othello dismisses Cassio from his service.
Desdemona has been awakened by the commotion, and Othello leads her back to bed, saying that he will look to Montano’s wound. Iago and Cassio remain behind, and Cassio laments the permanent damage now done to his reputation by a quarrel whose cause he cannot even remember. Iago suggests that Cassio appeal to Desdemona, because she commands Othello’s attention and goodwill. Iago argues that Desdemona’s kindheartedness will prompt her to help Cassio if Cassio entreats her, and that she will persuade Othello to give Cassio back his lieutenantship.
When Cassio leaves, Iago jokes about the irony of the fact that his so-called villainy involves counseling Cassio to a course of action that would actually help him. He repeats what he told Cassio about Desdemona’s generosity and Othello’s devotion to her. However, as Iago reminds the audience, he does the most evil when he seems to do good. Now that Cassio will be spending time with Desdemona, Iago will find it all the easier to convince Othello that Desdemona is having an affair with Cassio, thus turning Desdemona’s virtue to “pitch” (II.iii.234).
Roderigo enters, upset that he has been beaten and angry because Iago has taken all his money and left Roderigo nothing to show for it. Iago counsels him to be patient and not to return to Venice, reminding him that they have to work by their wits. He assures Roderigo that everything is going according to plan. After telling Roderigo to go, Iago finishes telling the audience the plot that is to come: he will convince Emilia to speak to Desdemona on Cassio’s behalf, and he will arrange for Othello to witness Cassio’s suit to Desdemona.