In the beginning of the play, Othello secretly marries Desdemona, the daughter of a Venetian Senator. The Turkish fleet attacks Cyprus and the Duke of Venice promotes Othello, putting him in charge of Venice's military forces. Othello must choose a lieutenant, and picks Cassio, an academic with no battlefield experience. Iago has fought with Othello and considers Othello's choice a slight to his loyal service. As revenge, Iago plots against Othello. He knows that the Moor suffers from terrible migraines and jealousy, and concocts a plan to convince Othello that Desdemona is sleeping with Cassio.
Iago knows that Cassio cannot hold his liquor. Once he has gotten Cassio drunk, Iago convinces Roderigo, a minor character, to pick a fight with Cassio. The ensuing melee causes a scene, and Othello strips Cassio of his rank. Cassio does not know that Iago plotted against him, and takes Iago's advice to appeal directly to Desdemona to regain his position as lieutenant. He meets with Desdemona for this purpose, not knowing that Iago plans to have Othello see their encounter.
In Act Three, Scene Three, Cassio meets with Desdemona at night to plead his case. When Cassio leaves Desdemona, Iago and Othello spot him but cannot be sure they saw Cassio. Othello asks "Was that not Cassio parted from my wife?" Iago takes care not to overplay his hand and instead makes passive-aggressive suggestions that it could not have been Cassio, hinting that Cassio would not need to act sneakily. This suggestion first plants the seed in Othello's mind that his wife has started an affair.
When Othello questions his wife about the visitor, she readily admits that it was Cassio and immediately pleads Cassio's case. Othello softens and says he will consider reinstating Cassio but then curses himself for giving in to Desdemona's charms. Iago continues his subtle hints; when Othello admits that Cassio acted as a go-between during his and Desdemona's courtship, Iago suggests that Cassio has pined for Desdemona all along. Othello demands that Iago talk plainly, but Iago continues to plant suggestions in Othello's jealous mind, even suggesting that Othello "beware jealousy, a green-eyed monster that mocks the meat it feeds on." Iago then reminds Othello that Desdemona betrayed her own father to marry him.
Near the end of Act Three, Scene Three, Othello begins suffering a migraine; Desdemona arrives and cools his brow with a handkerchief. Desdemona drops the handkerchief, which Iago's wife Emilia steals. An enraged and forlorn Othello returns to Iago and vows to kill Desdemona if he finds evidence of her infidelity. Iago gives this handkerchief to Cassio, who in turn gives it to his mistress Bianca. Othello sees Cassio give it to her. That night, he suffocates Desdemona with a pillow. When arrested for the crime, he laments that his tragedy stems from "not loving too wisely, but too well," asserting his own understanding of the tragic flaw that led to his demise.