In the imaginary land of Illyria, Duke Orsino is pining for the love of a countess. But the Countess Olivia has declared seven years of mourning for her dead brother and will see no one. And matters of love in Illyria are at a stalemate.

A sea storm washes upon Illyria's shore a young woman named Viola, who fears her twin brother Sebastian has drowned. Alone in a strange land, she takes on a young page's disguise, and as "Cesario" seeks employment in Duke Orsino's household. Charmed by his new page, Orsino immediately takes "him" into his confidence, and sends Cesario as his ambassador of love to the Countess Olivia. Viola undertakes Orsino's embassy, though she herself has fallen in love with the Duke. Cesario urges the Countess to drop her veil of mourning and, as she does, Olivia falls in love with the stranger before her.

Olivia's uncle Sir Toby Belch and his sidekick Sir Andrew Aguecheek—who also hopes to woo Olivia—drink late into the nights and the household is in an uproar. Olivia tries to maintain order through her steward Malvolio, hated by all. Her gentlewoman Maria plots their revenge—a forged letter of love in his mistress's hand that entraps Malvolio in his own desires. As instructed by the letter, he offers himself—adorned in yellow stockings—to his mistress, who in turn entrusts her steward to the care of her uncle.

As it turns out, Viola's twin is alive and well and in Illyria—looking just like his sister donned as Cesario. Olivia is overjoyed as a complete stranger consents to her proposal of marriage—stunning news to Viola when she learns of Cesario's impending wedding. With the appearance of the real Sebastian, all in Illyria is set aright. Almost.

Shakespeare borrows extensively for Twelfth Night from his earlier works, including The Two Gentlemen of Verona and The Comedy of Errors, and from earlier works of English fiction and Italian drama. His principal source was likely a tale told by the Englishman Barnabe Riche of "Apolonius and Silla," published in 1581 (about 20 years before Shakespeare wrote Twelfth Night). Silla is washed ashore where, disguised as the male page "Silvio," she enters into the service of Duke Apolonius, who loves Julina. Silla's twin brother (the real Silvio) arrives and, mistaken for his twin, becomes Julina's lover—before abandoning her pregnant, in search of his long-lost twin.

In 1537, an anonymous Italian comedy of disguise and mistaken identity was published, called Gl'Ingannati (The Deceived). A brother and sister are parted by accident, eventually to be reunited. The heroine Lelia dresses as the boy "Fabio" and finds that she must court the lady Isabella in the name of the master whom she herself loves. Isabella falls in love with "Fabio." When Lelia's long-lost twin brother Fabrizio, appears, he is arrested and locked up as a madman at Isabella's—who takes the opportunity to marry the person she mistakes for her "Fabio." Another play, Gl'Inganni, published in 1547, tells a similar story of disguise and secret love, and still a third play, by Curio Ganzaga, features a heroine named "Cesare," perhaps inspiring Shakespeare's "Cesario." Clearly love cloaked by disguise was a popular convention for theatrical productions of the time.

The subplot involving Malvolio, Maria, Aguecheek and Feste is Shakespeare's own addition to a complicated mix of disguise and identity confusion. According to Shakespearean scholar Anne Barton, the plot of Twelfth Night is "of the most ownerless and ancient kind: the very stuff of Comedy since Menander."

As Shakespeare wrote Twelfth Night, c. 1601, an aging and reclusive Queen Elizabeth remained on Englandıs throne until her death in 1603. Elizabeth had carefully avoided each marriage contract, which by aligning her with one faction or another might weaken her power. Now she and England faced her death with no heir apparent—making her country's future both uncertain and dangerous. Here at the heart of Shakespeare's final romantic comedy lives the Countess Olivia who, having lost her father and brother, seems determined to share her power with no one, turning away all male suitors—and in particular Orsino, the most powerful man in Illyria.

More than any other comedy of this period, Twelfth Night reveals its author's deep awareness of the shifting economic, social and political landscapes of his time. Elizabethan England was acutely aware of the possibility of upward mobility. The commerce of an ever-growing mercantile and industrializing world created "new money" and a new social class that "old money" viewed with disdain and fear. Malvolio suffers ridicule for his misguided attempt to marry "above his station," but he is by no means alone in his socially ambitious desires.

The promise in Twelfth Night's final scene of two well-matched marriages resolves this "dangerous situation" a great deal more easily than England itself could. But in his Olivia, Shakespeare portrays a woman who, having rejected all suitors, cloisters herself in a great household isolated from the world—then turns around and recklessly marries before she realizes the true identity of her suitor.

A barrister named John Manningham recorded in his diary that on February 2, 1602, he saw a performance of a play entitled Twelve night or what you will. Richard Burbage created the role of Malvolio, likely emphasizing the comic aspects of the character. The play proved popular, and by 1623 came to be known simply by the name of the audience's favorite character, Malvolio. During the Restoration, with the monarchy restored in 1660 and the theaters reopened, productions of Twelfth Night were not particularly well received. Elements of the play were incorporated into other works, but Shakespeare's script was not generally performed. By 1741, however, the play was once again an established favorite among London's theatergoers. The famous actor-manager David Garrick presented Twelfth Night at least once a season until the end of the eighteenth century at Drury Lane, now employing women in the female roles. Productions in the past 40 years have explored further the questions of gender and sexuality that Twelfth Night poses. Our evolving perception of gender and gender roles has also opened new understandings of the complex attractions between these characters.

Twelfth Night's appearances on the Chicago Shakespeare Theater stage exemplify the play's recent performance history. First performed at CST in 1996, the play was directed by British actor and director Michael Pennington, featuring Greg Vinkler as Malvolio. More recently, two companies have performed the play at CST with all-male casts: Shakespeare's Globe toured with its "original practices" production in 2003, with artistic director Mark Rylance in the role of Olivia; and Director Declan Donnellan and designer Nick Ormerod, joint founders of the British theater company Cheek by Jowl, teamed up with actors from the Pushkin Theatre of Moscow to create an innovative Russian-language version, which also toured here in 2006 as part of CST's World's Stage.