Here is a brief plot summary of Twelfth Night:
Despite his rejection Orsino sends his new page Cesario (Viola in disguise) to woo Olivia on his behalf. Viola goes unwillingly as she has already fallen in love at first sight with the duke. Olivia is attracted by the ‘boy’ and she sends her pompous steward, Malvolio, after him with a ring.
Olivia’s uncle, Sir Toby Belch, her servant Maria, and Sir Toby’s friend, Sir Andrew Aguecheek, who is also hoping to woo Olivia, and is being led on by Sir Toby, who is trying to fleece him of his money, all plot to expose the self-love of Malvolio. By means of a false letter they trick him into thinking his mistress Olivia loves him. Malvolio appears in yellow stockings and cross-garters, smiling as they have told him to in the letter. Unaware of the trick the Countess is horrified and has Malvolio shut up in the dark as a madman.
Meanwhile Viola’s twin brother, Sebastian, who has also survived the shipwreck, comes to Illyria. His sea-captain friend, Antonio, is a wanted man for piracy against Orsino. The resemblance between Cesario and Sebastian leads the jealous Sir Andrew to challenge Cesario to a duel. Antonio intervenes to defend Cesario whom he thinks is his friend Sebastian, and is arrested. Olivia has in the meantime met and become betrothed to Sebastian.
Cesario is accused of deserting both Antonio and Olivia when the real Sebastianarrives to apologise for fighting Sir Toby. Seeing both twins together, all is revealed to Olivia. Orsino’s fool, Feste, brings a letter from Malvolio and on his release the conspirators confess to having written the false letter. Malvolio departs promising revenge. Maria and Sir Toby have married in celebration of the success of their device against the steward.
The play ends as Orsino welcomes Olivia and Sebastian and, realising his own attraction to Cesario, he promises that once she is dressed as a woman again they, too, will be married.