If this be so . . .
. . .
Give me thy hand,
And let me see thee in thy woman’s weeds.
Orsino approaches Olivia’s house, accompanied by Viola (still disguised as Cesario) and his men. The Illyrian law officers come in looking for Orsino, dragging Antonio. Orsino, who fought against Antonio long ago, recognizes him as an honorable enemy. He asks Antonio what caused him to come into Orsino’s territory, where Antonio knew he would be in danger. Antonio responds by telling the story of how he rescued, befriended, and protected Sebastian, traveling with him to this hostile land. He lashes out at Cesario, whom he continues to mistake for Sebastian, claiming that Sebastian has stolen his purse and denied knowing him. Viola and Orsino are both bewildered, for Viola truly does not know Antonio.
Olivia enters and speaks to Cesario, she too believing him to be Sebastian, whom she has just married (at the end of Act IV, scene iii). Orsino, angry at Cesario’s apparent betrayal of him, threatens to carry Cesario off and kill him. Viola, resigned, prepares to go with Orsino to her death and says that she loves only him. Olivia is shocked, believing that her new spouse is betraying her. She calls in the priest, who, thinking that the young man in front of him is Sebastian, testifies that he has just married Olivia to the young man. Orsino orders Olivia and Cesario to leave together and never to appear in his sight again.
Suddenly, Sir Andrew enters, injured and calling for a doctor. He says that he and Sir Toby have just been in a fight with Orsino’s servant, Cesario. Seeing Cesario, Sir Andrew accuses him of the attack, but the confused Viola answers that she is not responsible. Olivia orders Sir Andrew and Sir Toby away for medical attention.
Finally, Sebastian appears, apologizing to Olivia for having beaten up Sir Toby and Sir Andrew. Recognizing Antonio, and not yet seeing his sister, Sebastian cries out joyfully how glad he is to see him. Dazed, all the others stare at Sebastian and Viola, who finally see one another. They interrogate one another with a barrage of questions about their birth and family history. Finally, they believe that they have each found their lost sibling. Viola excitedly tells Sebastian to wait until she has put her woman’s clothing back on—and everyone suddenly realizes that Cesario is really a woman.
Orsino, realizing that Olivia has married Sebastian, doesn’t seem terribly unhappy at losing her. Turning back to Viola, he reminds her that, disguised as a boy, she has often vowed her love to him. Viola reaffirms her love, and Orsino asks to see her in female garb. She tells him that her clothes were hidden with a sea captain, who now has taken service with Malvolio. Suddenly, everybody remembers what happened to Malvolio. Feste and Fabian come in with Malvolio’s letter, delivered from his cell. At Olivia’s order, Feste reads it aloud. Malvolio writes that the letter seemingly written to him by Olivia will explain his behavior and prove he is not insane.
Realizing that Malvolio’s writing does not seem like that of a crazy man, Olivia orders that he be brought to them. Malvolio is brought in, and he angrily gives Olivia the letter that Maria forged, demanding to know why he has been so ill treated. Olivia, recognizing Maria’s handwriting, denies having written it but understands what must have happened. Fabian interrupts to explain to everyone how—and why—the trick was played. He mentions in passing that Sir Toby has just married Maria. Malvolio, still furious, vows revenge and leaves abruptly. Orsino sends someone after Malvolio to make peace and find Viola’s female garments. He then announces that the double wedding will be celebrated shortly. Everyone exits except Feste, who sings one last song, an oddly mournful melody about growing up and growing old, and the play ends.