After the twins Sebastian and Viola survive a shipwreck, neither knows that the other is alive. Viola goes into service with Count Orsino of Illyria, disguised as a young man, "Cesario." Orsino sends Cesario to woo the Lady Olivia on his behalf, but Olivia falls in love with Cesario. Viola, in the meantime, has fallen in love with Orsino.

At the estate of Lady Olivia, Sir Toby Belch, Olivia’s kinsman, has brought in Sir Andrew Aguecheek to be her suitor. A confrontation between Olivia's steward, Malvolio, and the partying Toby and his cohort leads to a revenge plot against Malvolio. Malvolio is tricked into making a fool of himself, and he is locked in a dungeon as a lunatic.

In the meantime, Sebastian has been rescued by a sea captain, Antonio. When Viola, as Cesario, is challenged to a duel, Antonio mistakes her for Sebastian, comes to her aid, and is arrested. Olivia, meanwhile, mistakes Sebastian for Cesario and declares her love. When, finally, Sebastian and Viola appear together, the puzzles around the mistaken identities are solved:  Cesario is revealed as Viola, Orsino asks for Viola’s hand, Sebastian will wed Olivia, and Viola will marry Count Orsino. Malvolio, blaming Olivia and others for his humiliation, vows revenge.

Source: folger.edu