When Twelfth Night begins, Orsino, the Duke of Illyria, is lamenting his passionate but unrequited love for Olivia, who is mourning for her dead brother and has vowed to scorn the company of any man for seven years.

Meanwhile, Viola and the Captain land in Illyria. They have survived a shipwreck, but Viola is sure that her brother has drowned. 

The Captain assures her there is hope, for he saw her brother strap himself to the ship’s mast. The Captain explains the current situation in Illyria, including Orsino’s love for Olivia, and Viola resolves to serve the Duke disguised as a young man.

Sir Toby Belch, Olivia’s uncle, converses with Maria, her lady-in-waiting, about Sir Andrew Aguecheek, Sir Toby’s companion and another of Olivia’s willing suitors. Sir Andrew arrives shortly, and both “knights” engage in revelry and drunken foolishness. Sir Toby assures Sir Andrew that Olivia will not accept the Duke, for she will not marry above her station.

The Duke sends Viola, disguised as Cesario, to Olivia to plead his suit, even to the point of rudeness if necessary. The Duke hopes that Viola’s youthful innocence will appeal to Olivia. Still, Viola would rather be Orsino’s wife than woo Olivia on his behalf.

The fool Feste proves to Olivia that she is a fool for mourning to such extremes. Feste’s argument entertains Olivia, but her steward, Malvolio, is horrified by Feste’s brazenness. Olivia scolds Malvolio for his self-centered, opinionated sternness. Maria announces a young man at the gate. When Malvolio fails to send him away, Olivia agrees to see Cesario. 

Viola (as Cesario) delivers the Duke’s message of passion and suffering, but Olivia remains firm in her rejection. Olivia, however, is attracted to the “young man” before her. When Viola leaves, she sends Malvolio after her with a ring, supposedly to return it but actually to draw the “young man” back to her.

Act 2 opens with a conversation between Viola’s brother, Sebastian, who did not die, and his rescuer, Antonio. Sebastian will enter Illyria and seek the court of Duke Orsino. Now that his sister has drowned, he has nothing left but memories and sorrows. Antonio would like to follow, but he has enemies at the Duke’s palace.

In Illyria, Malvolio returns Olivia’s ring to the confused Viola, who realizes that Olivia has fallen in love with her disguise. Viola’s deception has created a knot she cannot untie, for she loves the Duke; the Duke loves Olivia, and now Olivia loves Viola’s male alter ego.

In the meantime, Sir Toby, Sir Andrew, and Feste engage in noisy revelry until Maria appears and tells them that Malvolio has threatened to throw them out. The three men taunt Malvolio, and the steward leaves in disgust, but Sir Toby desires revenge. Maria plans to drop a “love letter,” supposedly from Olivia, before Malvolio and then enjoy observing the consequences.

Viola-as-Cesario has returned to the Duke’s palace, where she discusses the nature of love with Orsino. The Duke proclaims that women can never love as strongly and passionately as men, and again he refuses to accept Olivia’s denial of his love. Viola tells him of her “sister,” who loved a man yet never told him of her feelings. The Duke sends Viola back to Olivia.

Maria initiates the love letter prank on Malvolio. Sir Toby and Sir Andrew, joined by another servant, Fabian, watch in hiding as Malvolio talks about his fantasies of power and honor, referring to himself as “Count Malvolio.” He finds the letter, solves its “riddle,” applies Olivia’s love to himself, and reads its instructions aloud. He is to wear yellow stockings with crossed garters; act with pride, disdain, and rudeness; immerse himself in his exalted status; and smile constantly. Maria and the three men enjoy this performance and expect more hilarity when Malvolio meets Olivia in person.

At the beginning of Act 3, Viola and Feste discuss the frivolity and wantonness of words, and Viola recognizes Feste’s wisdom. Viola then visits Olivia, proposing again on the Duke’s behalf. Olivia cares nothing for Orsino but admits to sending the ring after Cesario. Viola pities Olivia and warns her, “I am not what I am.” Still, Olivia declares her love for the “young man.” Viola, however, has only one heart and will never give it to a woman.

Sir Andrew overhears the conversation between Viola and Olivia and is furiously jealous. Fabian assures him, however, that Olivia only showed favor to Cesario as proof of her love for Sir Andrew. Sir Toby suggests that Sir Andrew compose an insulting challenge to Cesario. Just after Sir Andrew leaves to write his letter, Maria arrives to report that Malvolio is following his letter in detail.

The action then shifts back to Sebastian and Antonio. The latter has followed his friend into Illyria but will remain safely at the inn to avoid Illyrian officers. Antonio gives Sebastian his purse and money and sends him to enjoy the town’s monuments.

Meanwhile, Malvolio presents himself before Olivia, perfectly following the instructions in her supposed love letter, complete with the yellow stockings and crossed garters. Olivia thinks he has gone crazy. Malvolio does not notice her dismay and believes his hopes are coming to fruition. Sir Toby, Fabian, and Maria taunt Malvolio until he angrily departs. They then plan the next phase of their prank: they will lock Malvolio up as a madman.

Sir Andrew enters and reads his letter of challenge, which Sir Toby plans to deliver orally to frighten Cesario. Viola is meeting with Olivia, still insisting that the latter give her love to the Duke when Sir Toby issues the challenge. The bewildered Viola refuses it, for she worries that a fight will reveal her disguise. Antonio rushes in to defend Viola, whom he thinks is Sebastian, but Illyrian officers arrest him.

Act 4 starts with a confrontation between Sir Andrew and Sebastian, whom the former mistakes for Cesario. Olivia, summoned by Feste, invites Sebastian inside. Feste, now disguised as the priest Sir Topas, taunts Malvolio, who is locked in a dark house. Malvolio begs for light, paper, and ink to write a letter to Olivia pleading his case. Olivia proposes to Sebastian, who cannot decide if he is fortunate or crazy. The two go to a nearby chapel for their betrothal ceremony.

In the play’s final act, the Duke recognizes Antonio as his enemy, and Antonio, still thinking Viola is Sebastian, denounces Viola’s unfaithfulness. The Duke, however, is suspicious because Antonio claims three months’ acquaintance with the “young man,” and Orsino knows that Viola has been with him for the past three months. Olivia appears then and announces Cesario as her husband, which shocks both Orsino and Viola.

Sir Andrew rushes in, calling for a surgeon for himself and Sir Toby, for they have been attacked by Sebastian, who enters shortly. The company is amazed to see Sebastian and Viola side by side, and the siblings discover that they are both still alive. The Duke recognizes Viola’s womanhood, accepts her love, and proposes.

Malvolio, now freed from confinement, comes in, furious and accusatory. The prank against him is revealed, and Olivia promises justice, but he vows revenge on everyone. Malvolio exits, and the company follows him, hoping to “entreat him to peace.” Only Feste remains to perform a closing song.