Twins Viola and Sebastian are shipwrecked off the shores of Illyria. Both believe the other to be drowned. Viola is saved by a local ship captain. Finding herself in a strange land without her brother, she decides it prudent to disguise herself as a young man named ‘Cesario.’ With the help of the captain, she goes off to seek employment as a servant for a local Duke, Orsino.
Orsino has fallen in love with the Lady Olivia, who has promised to mourn the death of her brother for seven years. Passing off affairs of state to his assistants, Orsino spends most of his time at home, pining for Olivia and sending messengers to her in hopes she’ll return his affections.
Meanwhile, in Olivia’s house, her uncle Sir Toby Belch brings his wealthy but foolish friend Sir Andrew Aguecheek to woo Olivia. They are reprimanded by Olivia’s chambermaid, Maria, for drinking late into the night and partying in Olivia’s house in her time of mourning.
After Viola arrives at Orsino’s court, it takes only a few days for ‘Cesario’ to become one of Duke Orsino’s favorites. Given his new fondness for ‘Cesario,’ and believing his youthful charm will sway Olivia, Orsino sends the disguised Viola to woo Olivia on his behalf. Viola heads off to perform the task—despite the fact that she herself has fallen in love with Orsino.
At Olivia’s house, the family fool, Feste, has returned from a long absence and seeks to resume his job of telling jokes. Maria interrupts with news that Orsino has sent another messenger to Olivia. After outwitting her stern steward, Malvolio, Olivia grants ‘Cesario’ a private audience. As Viola works to convince Olivia of why Orsino is a worthy suitor, Olivia instead begins falling for ‘Cesario.’ After rebuking Orsino yet again, Olivia says that she will see no other messengers other than ‘Cesario’ and that she wants the youth to return the next day with news of how Orsino takes her rejection. After ‘Cesario’ leaves, Olivia sends Malvolio to bring a ring to the youth and say he left it behind.
Malvolio brings the ring to Viola (still disguised as ‘Cesario’) outside of Olivia’s home. Upon seeing the ring and hearing Olivia’s message, Viola works out her sticky wicket—she loves Orsino, who loves Olivia, who has fallen in love with her male alter ego, ‘Cesario.’
The situation is further complicated when Sebastian arrives in Illyria with Antonio, a pirate. Antonio rescued Sebastian from the shipwreck and they have since become close friends. Sebastian ventures to meet with Duke Orsino. Antonio, feeling a deep love for Sebastian, accompanies him despite being an outlaw in Illyria. For his own safety, Antonio gives Sebastian some money to spend in town, while he waits for him in their hotel.
Later, Sir Toby, Sir Andrew, Feste, and Fabian, another member of Olivia’s household, throw a raucous party late at night. There’s drinking, music, and mirth. Although she’s amused by Sir Toby and the lot, Maria attempts to quiet the group for Olivia’s sake. She’s followed by Malvolio, who harshly breaks up the party and threatens severe consequences if their revelry continues. Tired of Malvolio’s high-and-mighty attitude, Sir Toby, Sir Andrew, and Maria devise a plan to humiliate the steward.
Maria forges a letter to Malvolio, supposedly from Olivia, suggesting that she loves him and he should smile all the time and dress in a fashion that she, in actuality, detests. Malvolio, who is secretly in love with Olivia, does as the letter instructs. When he meets Olivia later, she is so concerned by Malvolio’s appearance and behavior that she asks Maria and Sir Toby to treat him for madness by locking him up.
Meanwhile, Sir Andrew laments Olivia’s continued favor toward ‘Cesario.’ He threatens to leave, but Sir Toby convinces him that Olivia, in fact, favors Sir Andrew and has been testing him by speaking with ‘Cesario.’ Sir Toby convinces Sir Andrew that he should challenge ‘Cesario’ to a duel to prove his valor and win Olivia. Just as Sir Andrew and ‘Cesario’ are about to fight, Antonio happens upon the scene and, believing the disguised Viola to be her brother Sebastian, intervenes and is arrested.
Later, Sir Andrew encounters the Sebastian on the street and re-issues his challenge, but Sebastian soundly beats him. Olivia arrives and mistakes Sebastian for ‘Cesario,’ asking him to marry her. He is immediately smitten by her and agrees.
Meanwhile, Malvolio is still imprisoned in the depths of Olivia’s home. With the help of Feste, Sir Toby and Maria continue to humiliate the steward. After he plays some tricks on him, Feste agrees to help Malvolio by giving him a pen, paper, and a light so that he can write a letter to Olivia.
The play comes to a head when Orsino, accompanied by ‘Cesario,’ finally comes to woo Olivia in person. Thinking she has married ‘Cesario,’ Olivia calls Viola her husband. Viola, of course, denies this union. Both Orsino and Olivia feel betrayed by ‘Cesario.’ Sir Toby and Sir Andrew enter and accuse ‘Cesario’ of assaulting them in the street. In the midst of all Viola’s denials, Sebastian arrives. The twins recognize each other, Viola is revealed as a woman, and they clear up the confusion.
In the midst of these revelations, Feste reveals Malvolio’s letter, which accuses Olivia of doing him notorious wrong. Sir Toby and Sir Andrew sneak away, leaving Fabian and Maria to confess the tricks they have played on Malvolio and reveal that Sir Toby has married Maria for her part in this ploy. Malvolio vows revenge.
At the play’s conclusion, Olivia and Sebastian remain wed and Orsino proposes marriage to Viola.