Viola and her twin brother Sebastian have been shipwrecked; each believes the other to be drowned. Viola disguises herself as a young man and, under the name of Cesario, enters the service of Orsino.
Orsino sends Cesario to woo Olivia on his behalf, but Olivia is taken in by Cesario’s disguise and falls in love with him.
Viola has secretly fallen in love with Orsino, and Orsino is confused by his feelings for his new ‘male’ servant. Meanwhile, Sebastian, Viola’s twin, has been saved by Antonio, who promptly falls in love with him.
Olivia’s butler, Malvolio, disapproves of all the other members of her household – her uncle Sir Toby Belch, his friend Sir Andrew Aguecheek and her servants, Maria, Feste and Fabia. Together, they plot Malvolio’s downfall. Malvolio is secretly in love with Olivia, and the others trick him into believing that these feelings are returned. To restrain his apparently insane delusion he is locked up.
Olivia meets Sebastian and, mistaking him for Cesario, asks him to marry her. He readily agrees. But when she later encounters Orsino and Cesario they are dismayed by her conviction that she and Cesario are betrothed. At this point, the twins are reunited and true identities revealed. Malvolio is released, and an attempt is made to pair everyone up satisfactorily.