Iagos plan worked, Cassio ended up getting into a fight on the job. Cassio has no idea whats coming for him...
Iago ends up telling Othello about what happened with Cassio on the job and Othello fires Cassio.
You're fired Cassio !!
Cassio goes to Desdemona for help to convince Othello to get Cassio his job back.
Oh Cassio! I will bring you up as much as I can to Othello !
Iago tells Othello that he thinks Desdemona is cheating on him with Cassio. Othello doesn't believe him until one day he saw that Cassio had his handkerchief. The handkerchief was given to Othello by his mother and he ended up giving it to Desdemona showing a symbol of love.
Sir, I saw Cassio with your handkerchief!!!
Othello confronts Desdemona about the affair with Cassio and how he got the hankercheif. Desdemona said it was all lies and that she would never do that to him. Othello doesn't believe her so he ends up strangling her to death!
But Othello I didn't !
Othello found out that Desdemona was telling the truth the whole time and that Iago was the one who was lying to him. Othello felt so guilt for killing Desdemona that he took his own life as well.