Othello: Novel Summary: Act 3 Scene 4

Desdemona still tries to plead Cassios cause, but Othello becomes very angry, and so she stops.  She is worried that she cannot find her handkerchief, and this makes Othello angrier.  While Cassio is biding his time, his wife Bianca arrives.  Cassio asks her to copy a handkerchief that has mysteriously been placed in his room.  This is, of course, the selfsame handkerchief of Desdemona, and Iago has put it there.