1.1 Iago announces that Othello has recently chosen Cassio for his second-in-command.
1.2 Cassio shows up with a message to Othello from the Duke: get over here.
2.1 Cassio is there to greet Desdemona, Othello, and Iago when they arrive at Cyprus. He is very friendly to them both and kisses Emilia hello, which does not please Iago.
2.3 After Iago gets him drunk, Cassio fights with Roderigo, and then with Montano, who's just trying to calm him down. Othello comes out, furious at the disorder, and fires Cassio. Iago suggests that Cassio turn to Desdemona to convince Othello to re-hire him.
3.1 Iago goes to Emilia and asks to be taken to Desdemona. She agrees.
3.3 Desdemona agrees to help Cassio get his job back. He's grateful, but slinks away when he sees Othello coming back.
3.4 Cassio comes in to see if Desdemona has succeeded in getting him his job back, but Desdemona and Othello have just had a fight, so it's not a good time. Later, Cassio gives Bianca (a prostitute he's involved with) a handkerchief he found in his room. Apparently he liked the pattern on it so much that he wanted Bianca to copy it. It happens to belong to Desdemona, stolen by Emilia on behalf of the scheming Iago. Trouble is a-brewing.
4.1 Cassio comes in and sees Othello in the middle of a fit. Iago assures him it's a normal thing and asks to talk to him later. Cassio comes back and jokes with Iago about Bianca and how needy and ridiculous she is. (Othello, overhearing, thinks they're discussing Desdemona.) Then Bianca comes in and throws the handkerchief back in Cassio's face; she's sure it's from some other woman, and who does he think he is!? Unfortunately, Othello is watching.
5.1 Cassio's minding his own business when Roderigo tries to attack him. Cassio beats him off, but Iago secretly wounds him from behind. Cassio falls and shouts for help.
5.2 Cassio, not dead, comes in after Othello has killed Desdemona to confirm that she never had an affair with him and that this was all Iago's terrible plot against them. While Othello has to go back to Venice to get punished for killing his wife, Cassio gets to take his (military) position in Cyprus.