Desdemona and Cassio speak about their mutual affection for Othello. Othello returns quickly, and Cassio leaves before he enters so that he can avoid the awkwardness of speaking to Othello. Iago capitalizes on this, and suggests to Othello that Cassio is acting suspiciously. Iago purposely gives false hints that he is keeping information from Othello, but gives Othello a warning against jealousy that feeds Othello's jealous fears. Othello starts to get a headache, and Desdemona offers to wrap his head with her handkerchief. When Othello is with her, all thoughts of jealousy leave him. Then she leaves, and accidentally drops her handkerchief. Emilia picks it up and gives it to Iago. Othello returns to speak to Iago, and his jealousy has returned once Desdemona has left his sight. He verbally attacks Iago, promising to kill him if he does not prove Desdemona's infidelity. Iago, now in possession of the handkerchief, makes up a story that he saw Cassio wipe his forehead with Desdemona's handkerchief after Cassio was dreaming sexually about Desdemona. Othello believes this as proof, and makes Iago his lieutenant as a reward for his 'honesty' and 'faithfulness." Iago says that he will kill Cassio, and Othello decides to kill Desdemona.
Othello: Novel Summary: Act 3 Scene 3