The play opens with Roderigo, a rich and dissolute gentleman, complaining to Iago, an ensign, that Iago has not told him about the secret marriage between Desdemona, the daughter of a Senator named Brabantio, and Othello, a Moorish general in the Venetian army. He is upset by this development because he loves Desdemona and had asked her father for her hand in marriage. Iago hates Othello for promoting a younger man named Michael Cassio above him, and tells Roderigo that he plans to use Othello for his own advantage. Iago is also angry because he believes, or at least gives the pretence of belief, that Othello slept with his wife Emilia. Iago denounces Cassio as a scholarly tactician with no real battle experience; in contrast, Iago is a battle-tested soldier. By emphasising Roderigo's failed bid for Desdemona, and his own dissatisfaction with serving under Othello, Iago convinces Roderigo to wake Brabantio, Desdemona's father, and tell him about his daughter's elopement. Iago sneaks away to find Othello and warns him that Brabantio is coming for him.
Brabantio, provoked by Roderigo, is enraged and will not rest until he has beheaded Othello, but even before Brabantio reaches Othello news arrives in Venice that the Turks are going to attack Cyprus; therefore Othello is summoned to advise the senators. When Brabantio arrives at Othello's residence, he is met by the messengers and guards of the Duke who keep him away from Othello. The Senator has no option but to accompany Othello to the Duke's residence where he accuses Othello of seducing Desdemona by witchcraft, but Othello defends himself before an assembly that includes the Duke of Venice, Brabantio's kinsmen Lodovico and Gratiano, and various senators.
Othello explains that Desdemona became enamoured of him for the sad and compelling stories he told of his life before Venice, not because of any witchcraft. The senate is satisfied once Desdemona confirms that she loves Othello and even declares that she will accompany him wherever he goes: "That I did love the Moor to live with him," but Brabantio leaves saying that Desdemona will betray Othello: "Look to her, Moor, if thou hast eyes to see:/She has deceived her father, and may thee." By order of the Duke, Othello leaves Venice to command the Venetian armies against invading Turks on the island of Cyprus, accompanied by his new wife, his new lieutenant Cassio, his ensign Iago, and Iago's wife, Emilia as Desdemona's attendant.
The party arrives in Cyprus to find that a storm has destroyed the Turkish fleet. Othello orders a general celebration and leaves to spend private time with Desdemona. In his absence, Iago schemes to get Cassio drunk after Cassio's own admission that he cannot hold his wine. He then persuades Roderigo to draw Cassio into a fight. The resulting brawl alarms the citizenry, and Othello is forced to quell the disturbance. Othello blames Cassio for the disturbance and strips him of his rank. Cassio is distraught, but, as part of his plan to convince Othello that Cassio and Desdemona are having an affair, Iago persuades Cassio to importune Desdemona to act as an intermediary between himself and Othello, in order to convince her husband to reinstate him.
Iago now persuades Othello to be suspicious of Cassio and Desdemona. Othello drops a handkerchief (with which Desdemona was trying to bind his headache) that was Othello's first gift to Desdemona and which he has stated holds great significance to him in the context of their relationship. Emilia finds it, and gives it to Iago, at his request, unaware of what he plans to do with the handkerchief. Iago plants it in Cassio's lodgings as evidence of Cassio and Desdemona's affair. After he has planted the handkerchief, Iago tells Othello to stand apart and watch Cassio's reactions while Iago questions him about the handkerchief. Iago goads Cassio on to talk about his affair with Bianca, a local courtesan with whom Cassio has been spending time, but speaks her name so quietly that Othello believes the two men are talking about Desdemona. Bianca, on discovering the handkerchief, chastises Cassio, accusing him of giving her a second-hand gift which he received from another lover. Othello sees this, and Iago convinces him that Cassio has received the handkerchief from Desdemona. Enraged and hurt, Othello resolves to kill his wife and asks Iago to kill Cassio. Othello proceeds to make Desdemona's life miserable, hitting her in front of visiting Venetian nobles.
Roderigo complains that he has received nothing from Iago in return for his money and efforts to win Desdemona, but Iago convinces him to kill Cassio. Roderigo attacks Cassio in the street after Cassio leaves Bianca's lodgings. They fight, and Cassio wounds Roderigo. During the scuffle, Iago comes from behind Cassio and badly cuts his leg. In the darkness, Iago manages to hide his identity, and when Lodovico and Gratiano hear Cassio's cries for help, Iago joins them, pretending to help Cassio. When Cassio identifies Roderigo as one of his attackers, Iago quietly stabs Roderigo to stop him from revealing the plot. He then accuses Bianca of the failed conspiracy to kill Cassio.
In the night, Othello confronts Desdemona, and then smothers her to death in their bed. When Emilia arrives, Othello tries to justify his actions by accusing Desdemona of adultery. Emilia calls for help. The former governor Montano arrives, with Gratiano and Iago, and Emilia begins to explain the situation. When Othello mentions the handkerchief as proof, Emilia realizes what Iago has done, and she exposes him, whereupon Iago kills her. Othello, belatedly realising Desdemona's innocence, stabs Iago but not fatally, saying that he would rather have Iago live the rest of his life in pain. For his part, Iago refuses to explain his motives, vowing to remain silent from that moment on. Lodovico, a Venetian nobleman, apprehends both Iago and Othello for the murders, but Othello commits suicide with a dagger he has hidden. Lodovico then declares Gratiano to be Othello's successor and exhorts Cassio to have Iago justly punished.