After Cassio is demoted, Iago tells him to go ask Desdemona to talk to her husband on his behalf. Meanwhile, Iago lures Othello towards them from a distance and suggests that they are getting too close.
Iago nefariously lies. He says he heard Cassio, in his sleep, talking about being with Desdemona.
Iago has his wife steal Desdemona's handkerchief, and plants it in Cassio's room.