Viola is Shipwrecked and Decides to Disguise Herself (Act 1, Scene 2): Viola, a young noblewoman, is rescued from a shipwreck by a sea captain. Viola assumes that she is the only survivor of the wreckage and laments the loss of her twin brother, Sebastian. The sea captain tells her that Illyria, the country she is in, is ruled by the noble Duke Orsino and that Orsino is currently romantically pursuing the Countess Olivia. Lady Olivia has sworn off the company of men as she mourns for her recently deceased brother. With the help of the sea captain, Viola decides to disguise herself as a man and enters Duke Orsino’s court. 

Orsino Sends Cesario to Woo Olivia (Act 1, Scene 5): Three days after the shipwreck, Viola has joined Duke Orsino’s household as a male servant named Cesario. Orsino dispatches Cesario to woo Lady Olivia on his behalf. Olivia initially refuses to meet with Cesario as she has vowed not to entertain any male suitors, but Cesario’s persistence wins her over. Olivia firmly rejects Duke Orsino’s proposal, but she finds herself attracted to Cesario and sends her steward Malvolio to give Cesario a ring on her behalf. Viola laments that circumstances have become so complicated, as she herself has fallen in love with Orsino. 

Malvolio is Deceived and Olivia Proposes to Cesario (Act 2, Scene 5 and Act 3, Scene 1): As retribution for Malvolio’s pompous attitude, several members of Olivia’s household decide to play a prank on him. Olivia’s servant Maria composes a letter designed to resemble Olivia’s handwriting, expressing love for Malvolio and instructing him to wear absurd clothes and behave ridiculously. Olivia’s uncle Sir Toby, her suitor Sir Andrew Aguecheek, and her servant Fabian spy on Malvolio as he discovers the letter and gleefully vows to follow its instructions. Meanwhile, Olivia proposes marriage to Cesario, who gently rejects her. 

Sebastien and Antonio Arrive in Illyria, Malvolio is Locked Up, and Sir Andrew Challenges Cesario to a Duel (Act 3, Scenes 3 and 4): Viola’s twin brother, Sebastian, arrives in Illyria with Antonio, who saved him from the shipwreck. Meanwhile, Malvolio follows the instructions from the letter he believes to have been written by Olivia. His actions scandalize Olivia, who has him locked up in order to regain his wits. Sir Toby then goads Sir Andrew into challenging Cesario to a duel for Olivia’s affections. Viola has never dueled before, on account of being a woman, but is unable to avoid the encounter. The duel is interrupted by Antonio, who mistakes Viola for Sebastian and intervenes. Antonio—who is considered a criminal in Illyria—is arrested, and the confused Viola ignores his pleas for help. 

Viola and Sebastian Reunite and the Confusion is Settled (Act 5, Scene 1): Orsino and Cesario go to Olivia’s house in order to confront Antonio and continue pursuing Olivia for Orsino. Antonio angrily remarks that Cesario, who he believes is Sebastian, has betrayed his friendship. Orsino and Cesario are greeted by Olivia, who addresses a bewildered Cesario as her husband. Orsino angrily rebuffs Cesario, who he believes has betrayed him by stealing Olivia’s affections. Sebastian’s arrival ends the confusion: the twins reunite and reveal that Olivia has actually married Sebastian, who she mistook for Cesario. Upon realizing that Cesario is actually Viola, a noblewoman, Orsino proposes to her and she happily accepts. The joyous atmosphere is interrupted by the arrival of Malvolio, who realizes he has been duped and swears revenge on the other characters.