In full Twelfth Night; Or, What You Will, a comedy in five acts, performed in 1601-02 and printed in the First Folio of 1623 from a promptbook or a transcript of it. One of Shakespeare's finest comedies, Twelfth Night precedes the great tragedies and problem plays in order of composition. The original source was probably the Sienese comedy Gl'ingannati (1531; "The Deceived"), published anonymously.

As the last of his three "mature comedies" (the other two being Much Ado About Nothing and As You Like It) and alike his early comedies, The Comedy of Errors or The Taming of the Shrew for instance, Twelfth Night is essentially a celebration of romantic love and can be viewed as a traditional romantic comedy. The play has many of the elements common to Elizabethan romantic comedy, including the devices of mistaken identity, separated twins, and gender-crossing disguise. its plot revolves around overcoming obstacles to "true" love. Like other representatives of the genre, Twelfth Night also features a sub-plot in which a self-inflated "sour" or "blocking" character, the steward Malvolio, is brought to his knees through a trick orchestrated by a ribald if also self-inflated character in the person of Sir Toby Belch.

However, unlike his early comedies, Shakespeare also strikes some discordant notes in Twelfth Night, including a conception of love and other themes that are not part of the conventional romantic comedy formula. Thus, for example, the subject of insanity surfaces as a salient theme and as a force within the plot. Indeed, while Twelfth Night concludes with tandem weddings, Shakespeare also speaks about the madness of love.

Off the coast of the country of Illyria, a storm has caused a terrible shipwreck. A young woman named Viola is swept onto the Illyrian shore. Finding herself alone in a strange land, she assumes that her twin brother, Sebastian, has been drowned in the wreck, and tries to figure out what sort of work she can do. A friendly sea captain tells her about a local nobleman, Count Orsino, who is currently courting a beautiful but reluctant noblewoman, Lady Olivia. Since Lady Olivia refuses to talk with any strangers, Viola can't look for work with her. However, she decides to disguise herself as a man, for protection, taking on the name of "Cesario," and goes to work in the household of Count Orsino.

Orisino is having little luck courting Olivia, who is in mourning for the death of her father and brother. Viola, as Orsino's proxy, is sent to Olivia with love letters. Viola refuses to leave until she is let in to see Olivia; Olivia, intrigued by the impudence of the "boy," contrives to get "Cesario" to return by sending her steward, Malvolio, after her with one of Olivia's rings. Viola realizes to her dismay that Olivia has fallen in love with her Cesario rather than Duke Orsino-proceedings are further complicated by Viola who is having stirrings herself for Orsino.

Now Viola (Cesario) is in a difficult situation. Viola loves Orsino, whilst Orsino loves Olivia, and Olivia loves Viola--and everyone is miserable.

Sebastian, who, as it turns out, is alive after all, arrives in Illyria, believing that it is his sister Viola. A man named Antonio had rescued him from the surf, and continues to aid him, at some personal risk, as Antonio in the past fought against the Duke. When Sebastian coincidentally approaches Olivia's grounds, cases of mistaken identity begin to pile up,

Meanwhile, in Olivia's house, Sir Toby Belch (Olivia's uncle) has conned Sir Andrew Aguecheek into supporting him, by convincing Aguecheek that he could be a suitor to Olivia. There has been a running feud between Malvolio and Belch; with the help of Maria, Olivia's maid, and Feste, a clown, Belch plots to make a fool of the steward. Maria writes a love letter to Malvolio that will make him think Olivia is in love with him.

Malvolio falls entirely for the ploy, which eventually leads to his confinement as a lunatic. All the while, Belch is egging Sir Andrew into a duel with Viola's "Cesario" character as she departs from Olivia; Olivia is now entirely smitten with Cesario, even though Viola continues to press Orsino's cause. As Viola and Sir Andrew prepare for a duel that neither one wants, Antonio happens upon the scene. Believing Viola to be Sebastian, he intervenes and is arrested. Viola, who, fails to recognise Antonio. Later, Belch and Sir Andrew encounter Sebastian, who doesn't back down from Aguecheek when challenged and resoundingly beats him. Olivia intervenes in the matter, and-mistaking Sebastian for Viola/Cesario-presses her suit for him. A bemused Sebastian agrees to marry her.

Antonio is brought before the Duke for questioning, and Viola relates the events of the duel. Antonio tells everyone how he dragged "this man" from the surf, saving his life. Then Olivia enters, searching for her new husband-which she thinks is Viola (as Cesario). Adding to this confusion, Belch and Aguecheek enter claiming that Viola/Cesario has violently assaulted them. In the midst of Viola's denials, Sebastian appears. The brother and sister recognize one another and are reunited; Sebastian helps to clear the confusion as to who fought and married who. At the end, Orsino and Viola pledge their love, Olivia and Sebastian will remain satisfactorily wed, and Olivia rebukes Belch and Maria for their abuse of Malvolio, who vows his revenge upon the whole lot. Belch agrees to wed Maria to make up for getting her in trouble, and all-except the disgruntled Malvolio-will apparently live happily ever after.

Finally, someone remembers Malvolio, and lets him out of the dark room. The trick is revealed in full, and the embittered Malvolio storms off, leaving the happy couples to their celebration.

The play pokes gentle fun at wooing and the folly of lovers. Duke Orsino, for example, is in love with the state of being in love, and Lady Olivia's initial vow of isolation to honour her dead brother is exposed as excessive and even self-indulgent. The two positions are contrasted with the mature and sensible attitude of Viola.