Viola “of Messaline”, wrecked on the Illyrian shore and believing wrongly that her twin brother Sebastian has been drowned becomes (in the male disguise of Cesario) a page to Orsino, the Duke. She bears his reiterated and scorned love message to the young countess Olivia, who is mourning affectedly for a dead brother. Olivia falls in love with Viola/Cesario. Meanwhile (Act II) Olivia’s parasitic uncle Sir Toby, her gullible suitor Sir Andrew, encouraged by Toby, her gentle woman Maria, her “allowed fool” Feste, and Fabian, also in her service, join to trick Malvolio, her somber, haughty and puritanical steward, an enemy of them all. “Dost thou think because thou art virtuous there shall be no more cakes and ale?” Toby asks him.  Presently, told by a forged letter (ostensibly Olivia’s, actually Maria’s) that Olivia is infatuated with him, Malvolio takes to himself the phrase: “Some are born great, some achieve greatness, some have greatness thrust upon ‘em.”

Obeying the false command to appear before his mistress smiling and in absurdly cross-gartered yellow stockings (“a fashion she detests”, “a colour she abhors”), Malvolio is carried off (Act III) to a dark cell as a presumed madman. Sebastian, whom we have realized by now was saved (believing his sister lost), has reached the town with his rescuer Antonio, a piratical captain who had once fought against Orsino’s ships. The plotters have persuaded Andrew, jealous of Olivia’s obvious love for Cesario, to challenge the page to a duel; while this is being scrambled though, Antonio arrives, mistakes Cesario for Sebastian, draw his sword to help, and is arrested by the Dukes’ officers.

Soon afterwards (Act IV), Toby, believing Sebastian to be Cesario, attacks him and is sternly rebuked by Olivia. Also mistaken, she begs the young man to go with her; he does so, pleasantly bewildered, and in a brief later scene she urges marriage (“Plight me the full assurance of your faith”) and they follow a priest to the chantry. Finally (Act V), confusions are resolved: the twins recognize each other; Viola, herself again, will be Orsino’s Duchess, his “fancy’s queen”; Toby weds Maria; Malvolio, released, swears revenge of “the whole pack of you”; and the comedy fades in Feste’s twilit song.

-from The Pocket Companion to Shakespeare’s Plays by J.C. Trewin