Lago assures Rodrigo that he will get Desdemona and him together as soon as the get revenge on Othello with this Cassio get the idea to start a rumor that Othello is cheating on his wife with Lagos wife Emilia and that Desdemona was cheating on Othello with Cassio this plan began as soon as they got to Cyprus.
Once they got to Cyprus they saw their was no battle just a party but Othello and Cassio were to keep watch anyway. Lago knowing this he gets Cassio drunk and instigates a fight between Cassio and Rodrigo. Othello sees the fight and fires Cassio for being drunk on the job. Lago then convinces Cassio to talk to Desdemona about asking Othello to give him his job back which was all apart of Lagos plan to plant the seed in Othello that Desdemona was cheating on him. Cassio goes over to Desdemona and talks to her about asking Othello to give him his job back and Desdemona agrees to do so but as Cassio and Desdemona are taking Othello approaches them and Cassio slinks away not wanting to make things awkward after just being fired. Lago who was with Othello as the walked up noted to Othello how supicious it was that Cassio left so quickly after being seen talking to Desdemona and hinted that Desdemona might be cheating on him. This thought makes Othello so upset that he becomes physically sick. Later on while Othello is still sick`Desdemona playfully bandages his head with a special handkerchief that he gave her but finds it too tight and lets it drop on the floor and he and Desdemona go to dinner. Emilia sees the handkerchief on the floor and stills it for Lago remembering him mentioning how bad he wanted it in the past. Lago plants the handkerchief in Cassio's room as "evidence" that Desdemona was cheating on Othello. Lago continues to hint to Othello that Desdemona is cheating on him so Othello gets fed up and demands proof. Lago mentions that he has seen Cassio wipe his face with Desdemona's handkerchief and show him the handkerchief in Cassio's room.