In Act III scene 1, Cassio enters with musicians with the intention of serenading Othello and Desdemona. A clown tells them that Othello will not want to hear more music. When the musicians exit, Cassio tells the clown to go and tell Emilia that he wants to see Desdemona. Iago enters and helps Cassio by getting Emilia and keeping Othello away. Emilia then enters and tells Cassio that Desdemona and Othello are discussing what happened between Cassio and Montano.
Act III scene 2 is a very brief scene where we see Othello carrying out his role and responsibilities. He gives Iago a letter to deliver to the senate and Othello, accompanied by gentlemen, walks along the castle battlements.
Act III scene 3 features the meeting between Cassio and Desdemona, who tells Cassio she will do what she can to help him. Cassio leaves as Iago and Othello enter and Iago suggests there was something underhand in Cassio's body language. Desdemona brings Cassio's case before Othello, who agrees to do what she wants. Left alone, Iago works more on Othello's jealousy, asking what he knows about Desdemona and Cassio. Emilia manages to take Desdemona's handkerchief which Iago then seizes and gives as proof of Desdemona's infidelity, saying that Cassio used it. Together they plot the death of Cassio.
In Act III scene 4, Desdemona realises the handkerchief is gone, but Emilia protests she does not know where it is. When Othello enters and asks for it, she believes Othello is ill. Cassio enters, pleading yet again for his suit to be resolved, but Desdemona tells him her husband is not well. She and Emilia go off to find him. Bianca enters complaining that she has not seen Cassio, and is suspicious about the handkerchief, which Cassio gives her.