Twelfth Night Plot Summary - By Akiva Fox, Literary Associate, Shakespeare Theatre Company

Duke Orsino of Illyria persists in courting the beautiful Countess Olivia, even though she has sworn off love to mourn for her deceased brother.  Meanwhile, a shipwreck separates a young woman named Viola from her twin brother, Sebastian.  Fearing for her safety, she disguises herself as a man and secures a position in Orsino's house.  Orsino soon sends the young "man" as his emissary to Olivia.
Olivia's Uncle, Toby Belch, disturbs her mourning, despite the efforts of Maria, Olivia’s attendant.  In order to finance his drunken binges, Toby has brought in the dim-witted Andrew Aguecheek.  Feste, Olivia's fool, also returns to the house to disrupt the mourning—much to the disapproval of Olivia’s steward, the sanctimonious Malvolio.
When Viola (now going by the name Cesario) arrives to woo Olivia for Orsino, the charming messenger unintentionally wins Olivia’s heart for “himself.”  The love-struck Olivia sends Malvolio after Cesario/Viola with a ring as a ploy to make him return the following day.  Viola immediately realizes that the countess has fallen in love with her male alter ego.  Elsewhere in Illyria, Viola’s brother Sebastian surfaces alive, believing his sister to be drowned. 
Toby and Andrew wake up the house with their late-night carousing, and Malvolio threatens them with eviction—on Olivia's authority.  Maria is outraged by Malvolio’s arrogance and vows to help Toby get his revenge.
Cesario/Viola attempts to make Orsino accept Olivia's rejection, nearly revealing her own unrequited love for him, but he sends her back to woo Olivia again.
Maria forges a cryptic love letter in Olivia’s handwriting, and Malvolio interprets it as an expression of Olivia's love for him.  He determines to follow its instructions—to wear yellow stockings and crossed garters, and to act boldly.  Cesario/Viola returns, and Olivia declares her romantic feelings. 
When Sebastian and his friend Antonio arrive in town, Antonio reveals that he once fought against Orsino and must hide until night.  He gives Sebastian his money for safekeeping.
Cross-gartered and in yellow stockings, Malvolio presents himself to a mystified Olivia.  She entrusts him to Toby, who orders him bound and imprisoned like a madman.  Toby next encourages a duel between the timid Cesario/Viola and Andrew.  Seeing what he thinks is Sebastian under attack, Antonio intervenes.  But Orsino’s officers arrest Antonio, and he feels betrayed when Cesario/Viola denies having his money.  The real Sebastian appears and is mistaken for Cesario, both by Toby and Andrew and by the amorous Olivia.  Attracted to Olivia, Sebastian impulsively agrees to marry her.
When Orsino arrives to court Olivia personally, Olivia not only rejects him but also calls Cesario her husband.  Andrew comes seeking help for Toby—wounded, Andrew claims, by Cesario.  Finally, Sebastian appears and apologizes to Olivia for injuring her uncle.  Reunited in the presence of the stunned assembly, Sebastian and Viola reveal that they are twins, brother and sister.  Olivia and Orsino accept the pair as their respective mates.  Olivia discovers the practical joke played against Malvolio, but he refuses to be reconciled as the others celebrate.