Othello, the Moor, is with Iago on another street in Venice. Iago is going on about how he's murdered a lot of people, but he really doesn't like to do it, because he's such an upstanding guy. Still, he tells Othello, he had a hard time not killing Brabantio, mostly because of the awful things he was saying about Othello. (Iago conveniently leaves out that he's the one who inspired Brabantio to trash-talk Othello in the first place. Oh, the treachery!)
Othello is calm, and says it's a good thing Iago didn't kill his father-in-law. Iago prattles on, asking if Othello's marriage is "fast" and "secure." It seems Iago is asking whether Othello's had sex with Desdemona yet, as marriages that had yet to be consummated (or sealed by having sex) could still be annulled. (We can safely assume that Desdemona is Brabantio's daughter.)
Iago is wagging in the Moor's ear that Brabantio is a Senator, and one who is so powerfully persuasive that he's almost twice as influential as the Duke of Venice. Iago is sure that Brabantio will try to have his daughter and Othello divorced, or otherwise raise hell for poor Othello.
Othello isn't going to stress about it. In fact, he's certain his record as a general and his service to Venice will stand up against any of Brabantio's complaints in Venice's eyes. Further, Othello says he simply loves Desdemona; he wouldn't have given up his freedom as a bachelor for anything less.
Their conversation is interrupted by some commotion. Iago assumes it's the warring Brabantio, and he encourages Othello to run off and hide, but Othello decides to stand and face his father-in-law like a man. Actually, the interruption is not the father-in-law mob at all, but Michael Cassio (the great arithmetician, and Othello's second-in-command) with many other officers.
Cassio brings the grave news that a dozen messengers have been coming with growing news of a war from Cyprus. The Duke of Venice and the Senate have been searching for Othello, who wasn't at his house. The Duke demands Othello's presence to deal with this matter immediately, even though it's the middle of the night.
Othello rushes off, leaving Cassio with Iago and just enough time for Iago to tell Cassio that the reason Othello wasn't around is that he was busy stealing away and getting married. Before Cassio can hear that Othello's new wife is Brabantio's daughter, Desdemona, Othello returns to get Cassio to go with him.
Just then, their exit is cut off by Brabantio, who's finally arrived with Roderigo and officers in tow. They're here to raise a riot against Othello. Swords are lifted all around. Othello and Brabantio's men plan to go at it with one another, but Othello stops them to ask what exactly Brabantio thinks he's doing. Brabantio explains that he's just come to clear up the little matter of Othello bewitching and stealing his daughter.
Brabantio says his daughter was so anti-marriage prior to meeting Othello that she wouldn't marry even the wealthiest boys in the kingdom. Brabantio is sure his daughter's sudden marriage to Othello is the result of witchcraft, and he'd like to have Othello locked up for practicing black magic.
Othello, still calm, says that even if he wanted to go to prison, it would only get Brabantio in trouble. The Duke is waiting to hear from Othello about this business with Cyprus, and he would be none-too-happy if his favorite general was locked up for marriage-related reasons.
Brabantio insists that his matter is important, too, regardless of what's keeping the Duke up at this hour. If the Duke or any of the other men of state knew a senator's daughter was out cavorting with a Moor and were comfortable with it, then all of Venice might have "bond-slaves and pagans" for their representatives – which is not a nice thing to say about Othello.