The play opens with Roderigo, a rich and dissolute gentleman, complaining to Iago, a high-ranking soldier, 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 lusts for Desdemona and has previously asked her father for her hand in marriage. Iago is upset with Othello for promoting a younger man named Michael Cassio above him, and tells Roderigo that he (Iago) is simply using Othello for his own advantage. Iago’s argument against Cassio is that he is a scholarly tactician and has no real battle experience from which he can draw strategy. By emphasizing this point, and his dissatisfaction with serving under Othello, Iago convinces Roderigo to wake Brabantio and tell him about his daughter’s affair. After Roderigo rouses Brabantio, Iago says aside that he has heard rumors that Othello has had an affair with his wife, Emilia. Later, Iago tells Othello that he overheard Roderigo telling Brabantio about the marriage and that he (Iago) was angry because the development was meant to be secret.
News arrives in Venice that the Turks are going to attack Cyprus; therefore Othello is summoned to advise. Brabantio arrives and accuses Othello of seducing Desdemona by witchcraft, but Othello defends himself successfully before an assembly that includes the Duke of Venice, Brabantio’s kinsman Lodovico and Gratiano, and various senators, explaining that Desdemona became enamored of him for the stories he told of his early life.
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 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. Iago schemes to use Cassio to ruin Othello and takes the opportunity of Othello’s absence at the celebration to persuade Roderigo to engage the drunken Cassio in a fight. The brawl greatly alarms the citizenry, and Othello is forced to quell the disturbance. Othello then strips Cassio of his rank. After Cassio sobers, Iago persuades him to importune Desdemona to act as an intermediary between himself and Othello, hoping that she will persuade the Moor to reinstate Cassio.
Iago now persuades Othello to be suspicious of Desdemona and Cassio. As it happens, Cassio is courting a woman named Bianca, who is a seamstress and, allegedly, a prostitute. Desdemona drops a handkerchief that was Othello’s first gift to her and which he has stated holds great significance to him in the context of their relationship; Emilia obtains this for Iago, who has asked her to steal it, having decided to plant it in Cassio’s lodgings as evidence of Cassio and Desdemona’s affair. Emilia is unaware of what Iago plans to do with the handkerchief. After he has planted the handkerchief, Iago tells Othello to stand apart and watch Cassio’s reactions while Iago questions him about the handkerchief. He goads Cassio on to talk about his affair with Bianca; because Othello cannot hear what they are saying, Othello thinks that Cassio is referring to Desdemona. Bianca, on discovering the handkerchief, chastises Cassio. Enraged and hurt, Othello decides he is going to kill his wife and Iago suggests to Othello to let him kill Cassio.
Iago convinces Roderigo to kill Cassio because Cassio has just been appointed in Othello’s place, whereas if Cassio lives to take office, Othello and Desdemona will leave Cyprus, thwarting Roderigo’s plans to win Desdemona. Roderigo attacks Cassio in the street after Cassio leaves Bianca’s lodgings and they fight. Both are wounded. Passers-by arrive to help; Iago joins them, pretending to help Cassio. Iago secretly stabs Roderigo to stop him from talking and accuses Bianca of conspiracy to kill Cassio.
In the night, Othello confronts Desdemona, and then kills her by smothering her in bed, before Emilia arrives. At Emilia’s distress, Othello tries to explain himself, justifying his actions by accusing Desdemona of adultery. Emilia calls for help. The Governor arrives, with Iago and others, and Emilia begins to explain the situation. When Othello mentions the handkerchief (distinctively embroidered) as proof, Emilia realizes what Iago has done; she exposes him, whereupon Iago kills her. Othello, realizing Desdemona’s innocence, attacks Iago but does not kill him, saying that he would rather have Iago live the rest of his life in pain. Lodovico, a Venetian nobleman, apprehends both Iago and Othello, but Othello commits suicide with a dagger before they can take him into custody. At the end, it can be assumed, Iago is taken off to be tortured and possibly executed.