So our cast of characters gets transported to Cyprus, where instead of battle there's just a big party (long story, read your play for the details). We note that Cassio is a ladies' man, especially around Emilia. While on watch together, Iago gets Cassio drunk and orchestrates a fight between him and Roderigo.
Othello intervenes and fires Cassio for being belligerently drunk instead of doing his job. Iago then convinces Cassio that he should ask Desdemona to tell Othello to give him back his job. Once alone, Iago schemes more about how he's going to convince Othello that Desdemona is having an affair with Cassio.