Explain Iago’s plan to get rid of Cassio and the ways in which he is able to manipulate Roderigo once again. What story does he tell Roderigo?

Upon leaving the women, Iago comes across Roderigo; he is not pleased with how Iago has handled things, and knows that although Iago is promising him Desdemona's favor, he has done nothing to indicate that he has worked to achieve this. Iago quiets him by making him believe that if he kills Cassio, then he will win Desdemona; Roderigo decides to go along with it, but Iago is coming dangerously close to being revealed.