Iago is a soldier who has fought beside Othello for several years, and has become his trusted advisor. At the beginning of the play, Iago claims to have been unfairly passed over for promotion to the rank of Othello's lieutenant in favour of Michael Cassio. Iago plots to manipulate Othello into demoting Cassio, and thereafter to bring about the downfall of Othello himself. He has an ally, Roderigo, who assists him in his plans in the mistaken belief that after Othello is gone, Iago will help Roderigo earn the affection of Othello's wife, Desdemona. After Iago engineers a drunken brawl to ensure Cassio's demotion (in Act 2), he sets to work on his second scheme: leading Othello to believe that Desdemona is having an affair with Cassio. This plan occupies the final three acts of the play.
