Shakespeare based his play on the story Un Capitano Moro (A Moorish Captain,) written by the Italian novelist and poet Cinthio. The term "moors" refers to Muslim inhabitants of the Arab west but was attributed to Arabs, Berbers, North Africans and Muslim Europeans.
The play begins with a jealous complaint, as Roderigo, a rich Venetian gentleman laments to his friend Iago about the secret marriage of Othello and Desdemona. Roderigo had asked Desdemona's father for her hand in marriage, but Othello's secretly married her, leaving Roderigo alone and upset.
Iago is Othello's ensign, and though trusted by Othello, he is jealous and spiteful as well. He believes he has been unfairly overlooked for promotion as Othello gave the young Michael Cassio a title above him. As both men lament their misfortune at the hand of Othello, they devise a plan to use Othello's misgivings to their advantage. Iago persuades Roderigo to wake the young Desdemona's father and inform him of Othello's secret wedding to his daughter.
The news infuriates Desdemona's father Brabantio and he vows to behead Othello, though he is prevented by Othello's residence that is full of the Duke of Venice's guards. In Venice, word has it that the Turkish plan to attack Cyprus. Othello is called upon to advise the senators, and Brabantio is left in the unfortunate position of accompanying Othello to the Duke's residence. His spite remains, and he accuses Othello of seducing his daughter with witchcraft.
Before the Duke of Venice, Othello defends his marriage to Desdemona, explaining that she fell in love with him when he told her the sad stories of his youth. Though the Duke and senate are satisfied after hearing Desdemona also proclaim her love for Othello, her father is dismayed. He tells Othello that she has deceived her father and will deceive him as well. Othello leaves Venice as commander of the Venetian armies, accompanied by his wife, lieutenant Cassio, Iago, and Iago's wife Emilia who is to serve as Desdemona's attendant.
Good news meets the band of travelers as they arrive in Cyprus to find that a storm has obliterated the entire Turkish fleet. Othello orders a celebration for all and leaves to spend time with his new wife. Iago, looking for trouble, intoxicates Cassio and encourages a fight between Roderigo and he. When Othello blames Cassio for the noise and strips him of his rank, Cassio is horrified. Iago suggests asking Desdemona to persuade her husband to reinstate his rank.
Iago plays all his cards, and persuades Othello that Cassio and Desdemona are not to be trusted. Emilia finds Desdemona's lost handkerchief, a gift from Othello, and gives it to her husband Iago. He continues the narrative of Desdemona's deceit against Othello with a series of lies involving Cassio and a local courtesan, the result of which are Othello vowing to kill his wife and asking Iago to kill Cassio. Iago shrugs off that responsibility to a still angry and misguided Roderigo.
As a scuffle ensues in the street, Iago first injures Cassio then kills Roderigo to stop him from reporting his plot. An enraged Othello smothers Desdemona to death in the bed believing she's committed adultery. Emilia realizes the handkerchief was alleged proof, and exposes her husband, who then murders her. Othello realizes he has killed his love in vain, and stabs, but does not kill Iago, preferring that Iago live the rest of his life in pain. Iago makes no explanation for his behavior, and after he and Othello are apprehended for their part in Roderigo and Emilia's murders, he commits suicide, leaving Iago to be punished solely by Cassio for his wrongdoings.