Act I
Scene 1: Iago enters in mysterious dialogue with Roderigo. They noisily wake Brabantio (Desdemona's father), Iago making Roderigo tell of Desdemona's elopement. Brabantio expresses grief/anger.
Scene 2: Othello learns from Cassio of a problem in Cyprus, while Brabantio wants to attack Othello, or arrest him. The Duke is in Council, they all go there.
Scene 3: The Council hear that the Turks seem to be attacking Cyprus, decide to give the command to Othello, who enters with Brabantio who complains his daughter has been bewitched. Othello's defence. The story of their meeting. Desdemona comes, explains her position between father and husband. Duke tries to make peace. Desdemona demands to go to Cyprus with her husband. Final dialogue between Iago and Roderigo, followed by Iago's soliloquy: "I hate the Moor."

Act II
Scene 1: The boats arrive in Cyprus, after a great storm, one by one, Othello last, after conversations between Desdemona and Iago etc.. Iago and Roderigo talk, Iago persuades Roderigo that Cassio should be removed. Soliloquy of Iago tells his plan to make Othello jealous of Cassio.
Scene 2: A proclamation of celebrations.
Scene 3: During the celebrations, Iago makes Cassio drink (he cannot take wine), Roderigo provokes a quarrel, Cassio wounds Montano who tries to restrain him. Othello restores order, asks Iago who started it, Othello takes Cassio's rank from him. Iago comforts Cassio, encourages him to ask Desdemona to intercede for him. Iago's "Divinities of Hell" soliloquy.

Scene 1: Interlude with musicians. Iago and Cassio talk, Emilia helps Cassio.
Scene 2: Othello, in charge of the city, goes walking.
Scene 3: Desdemona tells Cassio she will help him; Othello and Iago see Cassio leave her, Iago says, "I like not that." Desdemona asks Othello to take Cassio back, insists. Iago and Othello talk, bringing the destruction of Othello's trust (line 246 "Why did I marry?"). Desdemona comes to fetch Othello for the banquet, tries to bind his head (headache cure), drops handkerchief, Emilia picks it up, gives it to Iago. Othello returns to Iago, already believes everything, demands proof. Iago relates Cassio's dream, says he saw the handkerchief in Cassio's hands. Othello swears revenge, Iago swears to help. Othello asks him to arrange Cassio's death: "now art thou my lieutenant."
Scene 4: Desdemona and clown interlude. Desdemona says Cassio will come to see Othello, Othello asks for the handkerchief, describing it with magic details. Desdemona does not take him seriously: "The handkerchief!" Cassio, Iago, Emilia comfort her. Cassio with Bianca, Cassio asks her to copy the handkerchief he found in his room.

Act IV
Scene 1: Iago twisting Othello in all directions, Othello collapses. Cassio comes. Iago talks to him of women while Othello watches, and thinks that they are talking of Desdemona (only we hear both sides). Bianca brings in the handkerchief. Othello "I will chop her into messes." Messenger comes from Venice. Othello strikes Desdemona in public.
Scene 2: Othello questions Emilia, does not believe her. Othello confronts Desdemona. Iago and Emilia with Desdemona, various reactions. Roderigo challenges Iago about his promises; Iago suggests that Roderigo should kill Cassio.
Scene 3: Desdemona prepares for bed: the Willow Song.

Act V
Scene 1: The ambush of Cassio fails, Iago wounds him, he wounds Roderigo, Iago kills Roderigo, accuses Bianca.
Scene 2: The bedroom, Othello's soliloquy "Put out the light." Othello suffocates Desdemona. Emilia comes and announces Roderigo's death. Desdemona revives to speak, dies. Emilia confronts Othello: "My husband?" Iago and others arrive, Othello asks Iago to confirm the unfaithfulness of Desdemona, Emilia tells the truth about the handkerchief, Iago kills her. Othello made prisoner, his soliloquy "This look of thine will hurl my soul from heaven." Cassio and Iago brought in. Othello tries to kill Iago. Iago refuses to speak any more. Othello's makes last speech, kills himself "to die upon a kiss."

Here Shakespeare is again adapting an Italian novella, the story of an unnamed Moor and his wife Disdemona told by Giraldi Cinthio as the seventh in the third decade in his collection Hecatommithi (1566). A French translation was published in 1584; it is not certain which Shakespeare used. The novella gives the story of the Moor's love, the journey to Cyprus, and the destruction of the Moor's love by the cunning of his Ensign. The time scheme is much more relaxed, with the whole action covering a long period of time; the end of the story, the murder of Disdemona and its discovery, is unlike the play in almost every way.

In dramatizing a story that is centered on the romantic problem of "falling out of love," Shakespeare turns away from the socio- political chronicle sources underlying the other three great tragedies. In many ways he returns to techniques he used in Romeo and Juliet. In both plays the time-scheme is greatly condensed (from weeks to a few days or hours), although in both there are a few lines that suggest a double time scheme, a rhythm of events closer to the source that is useful dramatically. In both plays the audience is forced to watch the main characters making tragic decisions for lack of information that the audience has; the result is an element of suspense and horror unlike anything felt while watching Hamlet or King Lear.

The anonymous Moor of the source is given the name Othello, but it is striking to see that he is called the Moor, usually by Iago with much scorn, until the third scene when he is addressed as "Valiant Othello" by the Duke. Shakespeare leaves many things untold in this play, including how a Moor (from North Africa, black-skinned, normally a follower of Islam) comes to be a high general in the service of Christian Venice, fighting the Moslem Turks. In particular, the relationship between the blackness of Othello's skin and the tragedy that happens to him has been much discussed.

The play is an exploration of the way Othello's love for and trust of Desdemona is destroyed by the manipulations of Iago. The result is a frenzy of jealousy that can only be calmed by Desdemona's death. Critics often concentrate on the character of Iago, with his obvious links to the old morality plays' Vice. Once again, the question is Why? Why does Iago drive Othello to kill Desdemona? Coleridge made a famous remark about the "motive-hunting of a motiveless malignity" and certainly there are links with the destructive attitudes shown by Edmund in King Lear. The play is intensely concentrated; after the first act, the action proceeds without any obvious pause, and with no distraction. There is no subplot, no clowning, no fool. The whole play is summed up in the irony represented by the name given to Iago by almost every character: "Honest Iago."

One of the most terrible moments in the theater is that in which Othello prepares to murder Desdemona, assuring himself that he is doing the right thing:

It is the cause, it is the cause, my soul,
Let me not name it to you, you chaste stars:
It is the cause, yet I'll not shed her blood,
Nor scar that whiter skin of hers than snow,
And smooth, as monumental alabaster;
Yet she must die, else she'll betray more men.
Put out the light, and then put out the light:
If I quench thee, thou flaming minister,
I can again thy former light restore,
Should I repent me; but once put out thine,
Thou cunning pattern of excelling nature,
I know not where is that Promethean heat
That can thy light relume: when I have pluck'd the rose,
I cannot give it vital growth again,
It must needs wither; I'll smell it on the tree, (kisses her)
A balmy breath, that doth almost persuade
Justice herself to break her sword: once more:
Be thus, when thou art dead, and I will kill thee,
And love thee after: once more, and this the last,
So sweet was ne'er so fatal: I must weep,
But they are cruel tears; this sorrow's heavenly,
It strikes when it does love... (Act V.ii)

The entire action is complete, Desdemona is dead and "honest" Iago has fallen silent, before the characters in the play recognize their error in using this word; by that time Othello has believed Iago and turned his light into darkness.