We open with Othello grilling Emilia, trying to get her to confess that Desdemona and Cassio are having an affair. Emilia tells him that he's crazy—she has observed Cassio and Desdemona every minute they were together, and nothing remotely suspicious has happened. She's sure that Desdemona is honest, if ever there were an honest woman. Emilia insists that only some wretch could have put this thought into his head.
Othello then sends Emilia to get Desdemona, dismissing her claims as the simple testimony of a simple woman. Othello has convinced himself that Desdemona is cunning in her harlotry, and it's no surprise she wouldn't be found out, even by her woman friend.
Apprehensively, Desdemona enters. Othello flies into a passion, falling into tears. He accuses Desdemona of being false (particularly, by cheating on him), but Desdemona denies it and tries to argue otherwise.
She then suggests that Othello's rage might be inspired by the letter he received earlier today calling him back to Venice. Desdemona wonders if perhaps Othello thinks the summons to leave Cyprus (and her) were the machinations of her angry father back in Venice. Still, she says if her father had a hand in this, she's not to blame, as she remains staunchly on Othello's side.
Still, Othello mourns his mystery loss; he says he could bear any amount of suffering from the world, and proceeds to detail any and all types, including sores, poverty, slavery, and a world of scorn. With any of these, he says, he could have patience—but he cannot bear this abuse of his heart.
Desdemona begs him to tell her what she has done wrong, and Othello calls her a whore and a strumpet. Desdemona swears on her soul that she has never touched anybody but him, but he doesn't believe her.
Emilia walks in on this little exchange, so Othello takes to abusing her, too. He praises her for being the gatekeeper to Hell, and tells her that she'd do best to keep the events of this night to herself. Othello then exits, and the ladies are left with raised eyebrows.
Emilia questions Desdemona worriedly about Othello's behavior, wondering what's happened to her "lord." She then declares that she has no lord, nor does she have tears to cry, and no answer is appropriate about what is going on with Othello except an answer that could be told in tears.
Desdemona bids Emilia to lay her (Desdemona's) wedding sheets on the quarreling lovers' bed tonight, and asks to have Iago come and talk to her. Alone, she resents bearing all this abuse, mostly because she's done nothing wrong.
Emilia returns with Iago. Desdemona says she can't even begin to comprehend the things Othello has said to her. She was brought up so gently that she can't make sense of his abuse. Thankfully, Emilia witnessed the whole thing and is happy to dish. She says Othello called Desdemona a whore and all sorts of other cruel names—things worse than a drunk beggar would have said to a prostitute. Iago pretends not to know why Othello would behave this way and begs Desdemona not to weep.
Next, Emilia reminds Desdemona that she turned down all sorts of nice, rich Venetian boys, even her father, and her friends, and her country...all to marry Othello.
She also suggests that it could only be some really vile person, seeking his own self interest, that plied Othello with lies about Desdemona's faithfulness in order to make him jealous. She prattles on about this for a while, and Iago tells her to speak quietly, but Emilia notes that it was a very similar scheme, lies from a lying liar, that made Iago believe Othello had been with her too. Iago tells Emilia to shut up already.
Desdemona begs Iago to tell her what to do, or go talk to Othello on her behalf, to cure him of his wrong-mindedness. She can't believe this is happening to her—as she truly loves Othello. She can't even imagine going behind his back to be with somebody else.
Iago tells Desdemona not to worry—Othello is probably just upset about state business. He points out that the messengers from Venice are waiting to eat with the women, which is clearly more important than Othello's inexplicable and murderous rage. Iago promises "things shall be well" (4.2.170), and Desdemona and Emilia leave Iago alone.
Roderigo comes in to yell at Iago for not yet setting him up with Desdemona but still spending all of his (Roderigo's) money. Roderigo's finally starting to wise up to the fact that Iago is just using him for his cash, and in fact never really cared about him.
Roderigo, who seems rather broke at the moment, wants to know what happened to all the expensive jewelry he gave Iago to give to Desdemona. Iago kept promising that Desdemona was getting the gifts and wanted to give something up in return, but he has yet to see any special favors of Othello's wife.
Roderigo then throws down the gauntlet—he declares that he'll go and see Desdemona himself. If she returns his jewels, he'll repent ever having tried to court a married woman. But if she has no jewels to return, then Roderigo will take it out on Iago.
Iago, hearing Roderigo threaten him, declares him a much better man than he'd ever taken him for. Iago insists he's actually been working on the situation and that Roderigo will be all up in Desdemona's jewels come tomorrow night. All Roderigo has to do is listen to Iago's plan.
Iago informs Roderigo that Othello's been called back to Venice, and Cassio is to replace him in Cyprus. Iago also casts random lies, claiming Othello is headed to Mauritania (in Africa) with Desdemona. If Roderigo were to get rid of Cassio, then Othello couldn't leave Cyprus. (Presumably, this would give Roderigo access to Desdemona.)
Anyway, Iago quite expertly calms Roderigo down and convinces him that he needs to kill Cassio that very night, probably while Cassio is having dinner with his harlot (Bianca) who, it seems, forgave him for the whole handkerchief thing. Iago promises he'll be right behind him to help with the murdering. Iago declares all of this should go down sometime between midnight and one in the morning.
Iago is clear: murdering Cassio is the only way to get to Desdemona. Roderigo, ever a wit, points out that this plan really doesn't make any sense. Somehow Roderigo is appeased when Iago promises he'll explain it all later.