Along with Hamlet, King Lear, and Macbeth, Othello is one of Shakespeare's four great tragedies and thus a pillar of what most critics take to be the apex of Shakespeare's dramatic art. More than anything else, what distinguishes Othello from its great tragedies peers is the role of its villain, Iago. While the usurper King Claudius of Hamlet, the faithless daughters of Lear, and the unnatural villains of Macbeth (Macbeth, his Lady and the Weird Sister witches) are all impressively evil in their way, none of them enjoys the same diabolical role as Iago. Iago is a character who essentially writes the play's main plot, takes a key part in it, and gives first-hand direction to the others, most notably to the noble Moor, Othello. The play presents us with two remarkable characters, Iago and his victim, with Iago as the dominant force which causes Othello to see the infidelity of his young and beautiful wife, Desdemona, with his favourite lieutenant, Michael Cassio. Indeed, not only is "seeing" and the gap between appearance and reality a central theme of the play, it overlaps with other major thematic strands (trust, honour, and reputation) and sheds light on still others, including the theme of patriarchy and the political state.

In full Othello, the Moore of Venice is a tragedy in five acts, performed in 1604-05 and published in 1622 in a quarto edition from a transcript of foul papers. The First Folio version was taken from this quarto, with corrections from another authorial version of the play. The story concerns the destruction of a marriage by jealousy.

The plot of the play is set in motion when Othello, a heroic Moorish general in the service of Venice, appoints Cassio and not Iago as his chief lieutenant. Othello has just secretly married Desdemona, the daughter of Brabantio, a Venetian Senator. Iago enlists the aid of Roderigo, a rejected suitor, to tell Desdemona's father about the marriage. Brabantio goes to seize his daughter but is interrupted by news of a Turkish attack on Cyprus. The Duke and the Senate convene, and after hearing Desdemona and Othello testify to their love for one another, they allow her to accompany him to Cyprus, which he will defend against the Turks. Iago, whom Othello regards as honest and trustworthy, is given charge of Desdemona on the journey.

At Cyprus, Iago has Roderigo start a brawl, in which Cassio, Othello's lieutenant, wounds another man. Othello strips Cassio of his command and Cassio goes to Desdemona to ask her to convince her husband to reinstate him. Iago, meanwhile, sets about convincing Othello that Desdemona has been unfaithful to him--with Cassio. Through innuendo and pretence, Iago makes Othello almost mad with jealousy, until the Moor names him his lieutenant and makes him promise to help him kill Cassio and Desdemona.
Desdemona has lost a handkerchief that Othello gave her as a love-token, and Emilia, Iago's wife, finds it and gives it to her husband. Iago plants the handkerchief in Cassio's house, and then arranges for Othello to see Cassio give it to his mistress, Bianca. Othello is now convinced that Cassio has betrayed him with Desdemona, and that Cassio is flaunting his sexual conquest by giving Desdemona's handkerchief to a common whore. Othello begins abusing his wife in front of messengers from Venice, who are amazed to see this change in a man they thought to be noble.

That night, Othello tells his wife to wait for him in bed and goes out to walk about the city. Iago has convinced Roderigo that if he kills Cassio, Desdemona will sleep with him, so Roderigo attacks the former lieutenant. Cassio is saved by his mail shirt and wounds Roderigo; Iago flees, stabbing Cassio as he goes. Othello, passing, hears the cries and thinks that Cassio is killed, and so he returns to the castle to kill Desdemona. Meanwhile, Iago and the Venetian messengers find the wounded men; Iago secretly kills Roderigo and sends Emilia to the castle with news of the brawl.

Othello awakens Desdemona, tells her that she must die, and strangles her. Emilia returns and finds her mistress; Desdemona revives for a moment and then dies, saying as she passes away that Othello is innocent of her death. Her husband, however, proclaims his own guilt, and as the others return, Emilia realizes her husband's plot and exposes it. Iago, furious, stabs her and flees; he is captured and ordered to be tortured to death. Othello, heart-stricken, makes a final speech in which he passes sentence on himself, and then he commits suicide. He falls beside Desdemona.

The deep love between Desdemona and Othello is immediately evident. Again and again the moral and intellectual stature of Othello is emphasized. He quells tumults in the streets with a few words; he bears himself with dignity before the Venetian council, defending himself compellingly from bitter accusations by Brabantio and accepting his military burden with quiet confidence. Even Iago, in the opening scene of the play, grudgingly admits the dependence of the Venetians on his valour. After his terrible murder of Desdemona, Othello's contrition is agonizing enough to swing the sympathies of the audience back to him.

Trusting to false appearances and allowing reason to be guided by passion is a theme of many of Shakespeare's comedies. In Othello he showed the theme's tragic consequences. Shakespeare adapted the story from an Italian model by Giambattista Giraldi (Cinthio). His main innovation lay in developing the villainous character of Iago, whose motives are represented as complex and ambiguous.