Sir Toby convinces Sir Andrew to challenge Cesario to a duel in order to win Olivia’s heart. Antonio enters as the two are about to duel and thinks that Cesario is Sebastian, whom he cares about very much. He steps between the men and is carried away by guards because he is an enemy of Duke Orsino. However, because Cesario doesn’t recognize him, he believes Sebastian has betrayed him. Malvolio’s behavior scares Olivia, and she believes he has gone crazy. Maria, Sir Toby, and Fabian take the opportunity to lock Malvolio up.
Meanwhile, Sirs Toby and Andrew come across Sebastian nearby, and mistake him for Cesario. Andrew attacks Sebastian, and Olivia arrives, separating the men and bringing the very confused Sebastian back to her home because she thinks he is her Cesario, too. She asks him to marry her, and he happily agrees. Feste, the Fool, disguises himself as a priest and messes with Malvolio, making Malvolio begin to wonder if he is crazy after all.
Antonio is brought in to see Orsino, and his rantings about Sebastian’s betrayal are confusing to Cesario because he does not know him. Olivia comes in and greets Viola, thinking that she is the man that she just married. Sebastian arrives, and Viola reveals her true identity. Orsino comes to realize that he’s in love with Viola, and the couples are happy. Malvolio is released from his prison when it is revealed that the letter was a forgery; he swears his revenge.