The play opens at night, in the back streets of the wealthy Italian city of Venice, where Iago, who is a soldier under Othello's command, is busy tricking the gullible and lovesick Roderigo into believing he still has a chance of securing Desdemona's affection. He persuades Roderigo that Desdemona's love for the Moor is unnatural and cannot last, telling Roderigo to persist in his attempts to win Desdemona over by cunning means and promising, for a fee, to help in this task. He claims to be motivated by professional hatred of Othello for choosing a Florentine, Michael Cassio, as his lieutenant when he, Iago, was better suited to the post, and makes clear that he feigns loyalty to Othello only because this will enable him to mislead and undermine the general later.
The two then set about rousing Brabantio, Desdemona's father, informing him, in sexually crude and racist language, that his daughter has secretly eloped with Othello. Appalled to find his daughter is indeed missing from the house, Brabantio believes she must have been abducted and sets off to confront the Moor.
Meanwhile, Othello has been woken with news that the Turks, the enemies of the Venetians, have sent a fleet to invade the island of Cyprus, and that he should prepare to go to war at once. Brabantio and his men arrive and insist on taking Othello before the senate to answer the accusation that he has bewitched and stolen Desdemona; but when the Duke hears first Othello, and then Desdemona, describe the growth and extent of their love for one another, Brabantio's accusations are dismissed. Othello agrees to lead the Venetian defence of Cyprus, insisting that his new wife should be allowed to accompany him to the island. The extent of Othello's misplaced trust in Iago is underlined when he appoints him to take care of Desdemona on the sea voyage to Cyprus.
Having again cunningly persuaded Roderigo to continue his pursuit of Desdemona in Cyprus, Iago further reveals to the audience the depth of his own hatred for Othello. He despises the Moor's nobility and seems to be consumed by sexual jealousy, firstly because he too desires Desdemona and secondly because he suspects, on the basis of rumour alone, that Othello may have slept with his own wife, Emilia, Desdemona's servant. We realise he is grimly waiting for an opportunity to wreak havoc on the happiness of the newlyweds.
Iago's trump card is the handkerchief, a token of love given to Desdemona by Othello, which Desdemona has dropped by accident in an earlier scene though, crucially, she denies having mislaid it. Ensuring that Othello believes Cassio has the handkerchief, Iago is able to offer seeming proof of his liaison with Othello's wife. Now convinced beyond all doubt that Desdemona has betrayed him, he and Iago, whom he has now made his lieutenant, secretly vow to murder her and her supposed lover.
Cassio's murder is bungled, however, and though Iago kills Roderigo in an effort to silence him, letters are found on Roderigo's body that clearly implicate Iago. Meanwhile, bemused by the changes in her husband?s behaviour and protesting her innocence to the last, Desdemona is smothered to death by Othello in her bedchamber. When Emilia comes to inform her mistress of the attempt on Cassio's life, she is bewildered to find her murdered and summons others. Othello initially denies responsibility, then insists that the killing is justified because his wife was a whore. Only now does he learn from Emilia (who dies at Iago's hands for revealing the truth) how far her husband has deluded him. Appalled, he kills himself. Iago, refusing to give reasons for his treachery, is led away to be tortured.