Provoked by Iago, a jealous Roderigo tells Brabantio of Othello's secret affair and marriage to his daughter Desdemona. Othello is brought before the Duke, where he explains how he wooed Desdemona, and she is brought to the chamber to confirm her willingness to marry him.
Othello is then sent to defend Cyprus against the Turks. He prepares to leave, entrusting Desdemona to his ensign, Iago. Iago tells Roderigo that he will obtain Desdemona for him, and persuades him to come to Cyprus, along with his money. Iago reflects on his hatred for Othello, and plots a way of destroying him. Cassio, Othello's lieutenant, arrives in Cyprus, followed by Iago and Desdemona, and then by Othello.
The Turkish fleet is reported lost in a storm, and during the celebrations Iago persuades a reluctant Cassio to drink, then prompts Roderigo to start a fight with Cassio. Cassio becomes angry, and also attacks Montano, the Cyprus governor. Othello is roused, and removes Cassio from office. Iago advises Cassio to work his way back into Othello's affections by speaking to Desdemona. Cassio asks Iago's wife Emilia to arrange a meeting with Desdemona, and she agrees to do what she can. Iago puts the thought of Desdemona being unfaithful into Othello's mind, and hints at Cassio's role in the infidelity.
Emilia finds a handkerchief that Desdemona has dropped, and passes it to Iago. Othello's jealousy grows, and he asks Iago to prove his suspicions. Iago suggests that Desdemona has given Cassio the handkerchief. Othello asks Desdemona for it and is angered when she is unable to produce it. Cassio meanwhile has found the handkerchief in his chamber, and he asks his mistress Bianca to have it copied. Iago arranges for Othello to overhear a conversation between himself and Cassio, joking about Cassio's mistress Bianca - but making the conversation appear to be about Desdemona.
Bianca appears with the handkerchief and returns it to Cassio. This enrages Othello, and he hits Desdemona, much to the dismay of his kinsman Lodovico, who has arrived from Venice with letters ordering Othello to return. Othello asks Emilia about Desdemona's unfaithfulness, but she denies any knowledge of it. He then accuses Desdemona to her face, leaving her distraught. Iago convinces Roderigo that Cassio must die to keep Desdemona in Cyprus.
Roderigo attacks Cassio, but is wounded by him. Iago wounds Cassio. Hearing Cassio's cries, Othello thinks Iago has killed him, and leaves to deal with Desdemona. Iago then kills Roderigo.
Othello kills Desdemona, strangling her in her bed. Emilia discovers her body, and tells Othello that she was innocent, and that Iago had lied to him; she is stabbed and killed by Iago. Letters are revealed telling of Iago and Roderigo's plot to kill Cassio.
Othello, realising Desdemona was innocent, laments, then stabs Iago, but only wounds him. Lodovico gives Cassio power to rule in Cyprus, and is about to arrest Othello, when Othello commits suicide.