1) Iago works Othello into a trance. Othello, the once mighty orator of Act I, is hypnotized by jealous rage. Iago, the spider, has caught a prized spider.
2) Othello spies on Iago and Cassio. Iago has gotten Cassio talking about tupping Biancha, but Othello thinks he's talking about Desdemona.
3) The handkerchief appears in Biancha's hand. It has travelled from Egypt to Othello to Desdemona to Emilia to Iago to Cassio to Biancha, a prostitute. Othello knows he has lost Desdemona's love.
4) Lodovico arrives from Venice and gives Othello a letter that relieves Othello of his duties. He must return to Venice at once.
5) Othello slaps Desdemona. This is the first public show of anger on Othello's part. Lodovico and the Venetian emissaries are beside themselves. These two cooed over each other in Act I in Venice. And now this act of misogyny.