Cassio has explained the whole situation to Desdemona, and she promises to not rest until she's convinced Othello to reinstate Cassio as his lieutenant and renew their friendship. Cassio declares he's forever indebted to her, and Desdemona again emphasizes that she'll do everything she can. She even says, "Thy solicitor shall rather die/ Than give thy cause away" (3.3.27-28). Definition: foreshadowing.
Seeing Othello coming, Cassio decides it's time to leave. Desdemona tells him to stay, but Cassio feels too weird and hurries out. Thus, Iago begins his make-Othello-jealous campaign by commenting on how weird it is that Cassio hurried off so quickly, like a thief stealing away in the night.
Desdemona jumps right into sweet-talking Othello and campaigning for Cassio. She claims that Cassio is really sorry, and suggests Othello call Cassio back to plead his case. Othello says "not now," and Desdemona says something like, "well, maybe tomorrow, or Tuesday morning, or Tuesday night, or Wednesday morning, or how about Wednesday night?"
When Othello keeps putting her off, Desdemona claims she would never deny him anything, so why won't he listen to her? Besides, she has his best interests in mind.
Othello responds that he will deny her nothing, but in the meantime could she please leave him alone.
Iago asks fake-casual questions about Cassio, whom Othello says was often a go-between when he courted Desdemona. Iago keeps dropping uncomfortable hints, and finally, Othello demands to know what's bothering him. Iago says he'd rather not say, and then Othello presses him, and then Iago says he'd rather not say, and Othello presses.
Eventually, after Iago has cast doubt on Cassio's honesty, suggested he is disloyal, and hinted that Desdemona is unfaithful, Iago tells Othello, "O beware, my lord, of jealousy; It is the green-eyed monster which doth mock / the meat it feeds on" (18.104.22.168-165). That's great, except that by "beware" he really means "I hope you become jealous and kill your wife, because that would be ironic."
Othello says that he's not the type to get jealous—he builds his conclusions upon having suspicions, but only after he investigates them. If something seems wrong, he'll find out what's happening right away and resolve the situation. Othello knows Desdemona is desirable, and that he himself isn't, but that doesn't bother him. "She had eyes and chose me," Othello tells himself (3.3.187).
Further, Othello promises he has to see something to raise his suspicion before he'd have doubts about his wife's loyalty, and if he were to see and have doubts, he'd prove whether they were justified right away. This strategy helps him avoid being too influenced in his reason by love or jealousy. It's a good plan, except if it does exist, he doesn't use it.
Iago essentially says, "Okay, if you promise you won't be jealous, you should watch Desdemona with Cassio, but objectively, and not out of jealousy." Iago says he knows well the ways of the women of Venice: they are promiscuous, and though heaven knows their little exploits, their men don't.
Iago adds helpfully (to help with the objectivity bit) that Desdemona did deceive her father in order to marry Othello. He's implying, as Brabantio earlier did, that Desdemona is not to be trusted, as she is a woman, and thus a liar. Basically, Hamlet could get together with these guys and have a big anti-women party. (Frailty, thy name is lying scheming woman!)
Othello is persuaded by what Iago has said, and it's clear the seed of suspicion has been planted. Iago tells Othello that he hopes he hasn't ruined his day. This is the trickiest, most dastardly bit of all, because he totally hopes he has ruined Othello's day. Who's a liar now?
Othello assures Iago that he's a-okay, and Iago insists all of his speculations come from a place of love. Further, he tells Othello not to stretch out these suspicions into anything more serious or dangerous.
Othello promises he isn't much moved, and also, he may still think Desdemona is honest. "Still," Othello falters, "sometimes nature wrongs itself…" Iago seizes on this thought, playing painfully on Othello's insecurity. Iago claims it was against Desdemona's God-given nature to reject all of the suitors who came from her country, had her complexion, and her status.
Iago contends that Desdemona's unnatural choice against these suitors suggests that other "foul disproportions [and] thoughts unnatural" (3.3.231) might dwell in Desdemona's breast. Iago says he only brings this up to point out that one day, Desdemona might come back to her natural senses, repent her choice to marry Othello, and give him up for someone else less… well… black. (Note that Iago very carefully avoids explicitly saying that marriage to Othello in particular was an unnatural choice, but he exploits the heck out of the suggestion.)
On this despicable note, Iago leaves Othello to brood over the possibility that Desdemona is cheating on him, an undesirable black man. As Othello's busy wondering why he ever got married, Iago comes back to twist the knife a little more.
Acting regretful, Iago tells Othello not to think about it too much—it's probably nothing, he's probably overreacting, but just in case, Othello should keep an eye out for anything sketchy, especially anything like Desdemona seeming really keen on Cassio getting his position back. Iago once again leaves Othello to his thoughts.
Now alone, Othello praises Iago for being an honest man, a man insightful about matters of the heart. He then reflects on his relationship with Desdemona, using terminology from the gentleman's sport of falconry: in a rare moment of exquisite vulnerability, Othello compares Desdemona to his falcon.
He says if he does find that she is wild (haggard), then, though the leather straps that would tie her to his wrists (jesses) are actually his heart-strings, he would release her to fly on the wind at fortune, both "at random" and "to her fate," though he'd not know if she would ever return to him.
Othello undercuts this beautifully noble sentiment by thinking of other reasons Desdemona might be unfaithful to him. Whatever it is, Othello concludes Desdemona is lost to him; his only relief from his grief will now be to hate her. (A far cry from the falconry metaphor, we might note!)
Othello laments his position: men may say their women belong to them, yet they can never own their women's appetites for love and lust.
Still, Othello admits he could not bear to let others have even a little of his love's love. He decides this is the inevitable fate of important men: they are destined to be betrayed, even from the moment they're born.
Emilia and Desdemona come in, and for a moment Othello's mind reverses almost immediately; he can't believe that his wife would betray him. Desdemona has come to bring him to supper, along with the native Cypriots he's invited to dine. But he's not so ready for the partying; Othello's jealousy has already made him physically ill.
Desdemona notices that her husband seems unwell, and she tries to soothe him, offering to bind up his head with her handkerchief. Othello declares her handkerchief to be too little, and pushes it away from him. The two exit, leaving Emilia alone in the room.
Emilia's got her eyes on Desdemona's handkerchief, which Othello dropped during his angry moment. Emilia reveals that this was Othello's first love token to his wife, and her husband, Iago, has often asked her to steal it.
She hasn't been able to do so yet, as Desdemona loves it like a security blanket. Emilia decides to have the embroidery pattern copied, and then she'll give it to her husband, Iago. (Either handkerchiefs are pretty popular, or women must not have a lot of stuff do, because as you'll soon see, copying patterns of handkerchiefs is all the rage.)
She doesn't know what plans he has for it, but like a good wife, she'll make him happy, in the hopes that he'll love her in return.
Iago comes in and casually mocks his wife, as he usually does. Emilia, proud, produces the handkerchief. She admits she didn't steal it, but that Desdemona let it drop by negligence. Iago is pleased. He gives his wife a quick "good girl" before reverting to his usual rude self and telling her to hand it over. She asks what he intends to do with it before she hands it over, and Iago declares that's none of her business.
Emilia says that, her business or not, it had better be a good reason, as Desdemona will go mad once she realizes her favorite love token is gone. Iago instructs his wife to forget the whole incident. After he's grabbed the handkerchief, he orders her to leave.
Iago decides to put the handkerchief (which he also calls a napkin) in Cassio's room, in order to fuel Othello's suspicions. Though the token is only a little thing, it's enough of a confirmation to set off Othello's jealous fantasies about what Cassio might be doing with the handkerchief (and his wife).
Watching Othello enter again, Iago gloats that none of the drugs in the world could make the man rest easy, now that he's worried about his wife. Othello takes his own turn to curse; he declares it's better to be greatly wronged and know about it than to have just a bit of suspicion that one is wronged.
Iago acts confused, as though he doesn't understand that Othello is angry at him for planting the seed of suspicion. Othello declares that he would never have suspected Desdemona before, since a man who's robbed of something and doesn't know it is as good as not having been robbed at all. Basically, even if Desdemona is sleeping all over Cyprus, Othello would be better off not knowing about it.
As it stands, knowing of Desdemona's potential unfaithfulness has destroyed Othello's own identity. His peaceful mind, his happiness, and even his standing as a soldier (especially his joy in the glory of war) are all tainted by this knowledge. Desdemona has unmanned him, and even Othello's proud victories on the battlefield now seem out of reach.
He demands that Iago give him proof of Desdemona's cheating on him. If Iago is just playing with him, he's going to really regret it.
Iago acts all insulted that Othello doesn't trust him, and cries out that it's an awful world where one can be punished so much for their well-intentioned honesty. Othello agrees Iago should be honest, and again demands that he bring him proof of Desdemona's infidelity.
Iago describes how difficult it would be to prove infidelity; would Othello like to see Desdemona and Cassio caught in the throes of passion? Iago then painstakingly conjures an image of Desdemona and Cassio being passionate together, and says Othello wouldn't like to see that, now would he? The power of the mental image is key here.
After this vivid description, Othello again presses Iago for some good reason why he should suspect Desdemona. Iago describes how at one time he did "lay with Cassio" (as manly bunkmates). Iago was kept up by a toothache, and he was awake to hear, he claims, Cassio mumbling in his sleep.
Iago recounts that Cassio supposedly called out to Desdemona in his sleep, telling her to be cautious and hide their love. Then Cassio started writhing around in the bed and kissing Iago's hand as if it were Desdemona. At this point, we're kind of confused as to why Iago wouldn't wake Cassio up and say, "Excuse me, please stop making out with my hand."
Anyway, the still-asleep-and-dreaming Cassio then supposedly threw his legs over Iago's thighs, kissed some more, and finally declared, "Cursed fate that gave thee to the Moor!" (3.3.423), in case Iago had left any doubt.
Othello, not surprisingly, is furious, but Iago is quick to note that this was all just Cassio dreaming, a highly incriminating dream, no doubt, but a dream nonetheless. Regardless, Othello is totally convinced by this story about Cassio in bed.
With great timing, Iago puts the final nail in the coffin. Still playing innocent, and instructing Othello to be calm (which only fires his fury), he tells Othello he saw Cassio with Desdemona's special handkerchief.
Hearing this, Othello announces all his love for Desdemona is gone. He's now out for blood and revenge, hopefully in one convenient package. Iago, hearing this, makes some paltry attempts to remind Othello that they've only got suspicions, but again, this only sharpens Othello's desire for revenge. Othello then kneels and swears that his former love for Desdemona will not stop him from bloodily avenging her betrayal.
Iago kneels with him and vows to do whatever it takes to help regain honor. Othello asks him to kill Cassio. Iago agrees and slyly adds, "but let her live" (3.3.471), speaking of Desdemona. "Damn her, lewd minx!" Othello curses. Othello's decided that she has to die. To close the scene, Iago declares, "I am your own for ever" (3.3.479), actually meaning that he's totally owned Othello.