What is the plot of Twelfth Night?

Twelfth Night is a story about transgression. Shakespeare
plays with the themes of love, mistaken identity, and social class in this comedy. The
play actually contains three plotlines that come together in the final scene.  The
plotlines are held together by the character of Feste, the Fool, who can cross social
boundaries because of his exemption from behaving, the right of an "allowed Fool." The
plotlines are as follows:

1. Viola, stranded on Illyria
after a shipwreck and the loss of her twin Sebastian, is forced by her status as a
single, unaccompanied woman, to disguise herself as a boy and work at Orsino's court.
There she falls in love with Orsino but cannot reveal herself because of her deception.
Orsino is in love with the Lady Olivia, who has also recently lost a brother. He sends
Viola, in her disguise as the page Cesario, to woo Olivia. Olivia then falls in love
with Cesario, unaware that "he" is "she."

2. Sebastian did
not drown, but was saved by Antonio, who cares for him. He, too, is in Illyria, but does
not know that his sister is alive. He and Antonio must part ways since Antonio is
Orsino's enemy, but he does not want to leave Sebastian and follows
him.

3. Toby, Olivia's relative, has brought Andrew, a
"foolish knight" to woo her in hopes of being the financial beneficiary of their union.
They are continually in trouble for drinking and partying at all hours in Olivia's house
with the help of Maria, Olivia's gentlewoman.  The steward Malvolio, who has delusions
of marrying Olivia himself, threatens to expose them and they devise a plot to undo him,
sending him to Olivia under the pretense that she has written him a
letter.

Of course, all the characters finally come together
and are unmasked at the end of the play. Orsino gets Viola; Sebastian gets Olivia, and
Maria marries Sir Toby.