Cassio tried to be in good terms with Othello by using musicians but fails. Cassio then talks with Iago about the plan to draw out Othello from Desdemona and then talks with Emilia about having a meeting with Desdemona to get Cassio's job back.
Othello orders Iago to deliver the letters to the Senate. Othello also inspects the fortifications.
Desdemona promises Cassio to give him an audience with Othello. As Othello approaches, Cassio flees the scene which Iago uses this as an excuse to get Othello suspicious about the interaction between Cassio and Desdemona.