Viola and her twin brother Sebastian have been shipwrecked off the coast of Illyria and each believes the other to be drowned.
Viola disguises herself as a boy and, under the name of Cesario, enters the service of Duke Orsino. The Duke sends Cesario to woo the lady Olivia on his behalf but Olivia falls in love with the attractive 'boy'. Sebastian is saved by the sea captain Antonio and arrives in Illyria.
Malvolio, Olivia's steward, disapproves of the other members of her household - her uncle Sir Toby Belch, his friend Sir Andrew Aguecheek and the jester Feste. Together with Maria, Olivia's waiting-woman, these three plot Malvolio's downfall.
Olivia meets Sebastian and, mistaking him for Cesario, arranges for them to be secretly married. Other people are also confused by meeting 'Cesario' unexpectedly.
Orsino is furious at the apparent falseness of his page, but with the meeting of the twins, true identities are revealed and Orsino recognises his love for Viola.
