Despite feeling feint of heart, Roderigo resolves to kill Cassio if only for the sake of preventing Othello from leaving Cyprus right away with Desdemona in tow, which he assuredly will (to attend to his father-in-law’s funeral) if nothing is done to bring the affairs of state to the point of a crisis. (By killing Cassio, Roderigo would create a vacuum in the chain-of-command which would compel Othello, and by extension Desdemona, to linger in Cyprus.) Roderigo’s ambush proves to be a disaster, however, as Roderigo sustains a serious wound due in large part to the thickness of Cassio’s coat. Worse, Roderigo is betrayed by Iago who, having determined that the mutual deaths of Cassio and Roderigo are the ideal outcomes (of the ambush), slays Roderigo. Luckily for Cassio, circumstances limit the harm that befalls him to a body wound. Attacked from the rear, Cassio is ignorant of his assailant, but Othello correctly surmises that Iago is Cassio’s assailant upon hearing Cassio’s dire yell in the night. Praising Iago’s loyalty, Othello goes to see about Desdemona. Meanwhile, Cassio’s welfare is attended to by Iago, Lodovico, Gratiano, Bianca, and Emilia.
Awakened by Othello’s arrival, Desdemona anticipates her husband to be joining her in bed only to be alarmed by his strange words and--moreover--by the expression on his face which denotes a grimness of purpose from which there is no appeal. When asked to explain his black mood, Othello alleges Desdemona of infidelity and perjury which allegations Desdemona categorically denies but to no avail. Othello means to kill her and kill her now. In vain, Desdemona pleads for her life.
Subsequently, Emilia arrives to inform the Moor of Roderigo’s death and Cassio’s injury when she discovers that Desdemona is dead. Desdemona’s dying voice informs Emilia that Desdemona's death was self-afflicted and that Desdemona's final thoughts are for her husband--Othello--whom she--Desdemona--will forever love. Addressing Emilia, Othello questions the credibility of Desdemona’s dying voice. When Emilia says that she won’t contradict it, Othello divulges that it was in fact he who had killed Desdemona. Emilia denounces Othello who defends himself, citing Emilia’s husband as the source of his intelligence which intelligence proved Desdemona to be lecherous and unfaithful. Incredulous, Emilia cries murder at the top her voice drawing Montano, Gratiano, and Iago to the scene. Addressing Iago, Emilia pleads with him to deny Othello’s claim that it was he--Iago--who called Desdemona’s faithfulness into question. Iago confirms Othello’s claim, however, arguing that his suspicion of Desdemona’s unfaithfulness was based entirely on evidence. When Othello justifies Iago’s suspicion by mentioning Desdemona’s handkerchief, of its being in Cassio’s possession, Emilia proves to Othello that his understanding of how the handkerchief got into Cassio’s hand is patently false, that Iago had made it seem as if Desdemona had given it to Cassio as a token of love. Consequently, Othello attacks Iago only to be restrained and disarmed. In the skirmish, Iago flees but not before fatally wounding his wife. Montano pursues Iago and Othello mourns for Desdemona.
Eventually, Iago is caught and brought before Othello. Also present are Lodovico and Cassio. Once again Othello attacks Iago and once again Othello is disarmed. Though wounded this time, Iago will live. Othello asks why, why has Iago done what he has, but Iago refuses to say. Lodovico avows that Iago will be prosecuted to the full extent of the law and then some. The evidence in support of Iago’s guilt is overwhelming. As for Othello, he is to be divested of his command and his case brought before the Venetian state for review. Lodovico thus concludes the proceedings when Othello delivers an impromptu speech, wherein he urges he be remembered as someone foolish and ignorant who has discarded life’s greatest treasure but for all that someone deserving of some pity and praise, before stabbing himself to death. Kissing Desdemona, Othello dies.