Duke Orsino is in love with the Lady Olivia, but she does not love him. A servant brings Orsino a message from Olivia saying that she can never love him and is going to mourn for seven years in ‘sad remembrance’ of her brother who has recently died.

Viola was travelling with her twin brother, Sebastian. They were separated when their ship got into trouble and sank at sea and she believes her brother has drowned. Viola is washed ashore in Illyria with some of the ship’s crew. The ship’s captain agrees to help her dress as a boy and get a job with the governor of Illyria, Duke Orsino.

Sir Toby Belch is Lady Olivia's uncle and he lives in her house. Olivia's maid Maria tells Sir Toby that her mistress is tired of his ‘quaffing and drinking’ and is not impressed with his friend Sir Andrew Aguecheek, who he brought round to woo Olivia. Sir Toby encourages Sir Andrew to believe Olivia might marry him and the two of them go off to spend their time drinking and dancing.

Viola has disguised herself as a boy and named herself Cesario. She has been working in Orsino’s house and he has told her all about his love for Olivia. He sends 'Cesario' to Olivia to ‘unfold the passion of my love’, but Viola tells the audience that she wishes she could marry Orsino herself.

Viola follows Orsino’s orders and refuses to leave Olivia’s gate until she is allowed to speak to ‘the lady of the house’. Olivia lets ‘Cesario’ in but insists that she does not love Orsino. After 'Cesario' leaves, Olivia confesses to the audience that she has fallen in love with the messenger and sends 'Cesario' a message via her head servant Malvolio.

Viola’s brother, Sebastian, was rescued from the sea by Antonio, who has grown very fond of him. Sebastian tells Antonio that he believes his twin sister drowned in the shipwreck. He leaves Antonio, saying that he is ‘bound to the Count Orsino’s court’. Antonio has enemies there but decides to follow Sebastian anyway.

Malvolio catches up with 'Cesario' and gives 'him' a message from Olivia: that she is not interested in Orsino’s love or his ring. Viola knows she ‘left no ring’ and quickly realises what has happened, telling the audience ‘I am the man’ that Olivia has fallen in love with.

Sir Toby and Sir Andrew are up ‘after midnight’ and have been drinking. Feste, Olivia’s jester, joins them and Sir Toby suggests they ‘rouse the night-owl in a catch’. They wake up Maria, Malvolio then arrives and orders them to be quiet. Sir Toby is offended. When Malvolio leaves, Maria promises they can get their own back by making him think that Olivia loves him.

Orsino talks to 'Cesario' about love and, while in disguise, Viola describes her own feelings. It becomes clear that Viola is talking about her love for Orsino, but Orsino assumes 'Cesario' is talking about a woman. Orsino tells 'Cesario' to go to Olivia again.

Maria tells Sir Toby, Sir Andrew and Fabian to hide so they can spy on Malvolio. When he finds and reads Maria’s letter, Malvolio is completely convinced it is a love letter from Olivia and declares he will do everything she asks him, including wearing ‘yellow stockings’ and smiling.

Olivia comes out to meet ‘Cesario’ as he approaches her house and insists on being left alone with ‘him’. At first, Olivia seems to accept that ‘Cesario’ does not return her love but then declares ‘I love thee so, that maugre all thy pride, / Nor wit, no reason can my passion hide’ and she asks 'him' to visit her again.

Having seen how much attention Olivia gives to ‘Cesario’, Sir Andrew realises that Olivia has no interest in marrying him and decides ‘I’ll not stay a jot longer’. Sir Toby and another servant Fabian have fun convincing him that Olivia wants to make him jealous and he agrees to challenge 'Cesario' to a duel. Maria then arrives to tell them Malvolio is about to go to Olivia wearing yellow stockings, a colour she hates.

Sebastian is pleased to see ‘kind Antonio’ again and suggests they set out to see the town together. Antonio confesses that he is a wanted man in this town. He gives Sebastian his purse in case his eye ‘shall light upon some toy / You have desire to purchase’ and arranges to meet him later at an inn called the Elephant.

Olivia sends for Malvolio and is very confused to see him dressed in yellow stockings and smiling. As she leaves to meet ‘Cesario’, she tells Maria ‘let this fellow be looked to’ and asks her to call Sir Toby for help. Sir Toby, Maria and Fabian treat Malvolio as if ‘the fiend speaks within him’ and he tells them ‘Go hang yourselves all’.

Sir Andrew returns with the letter he has written challenging ‘Cesario’ to a duel. He thinks ‘there’s vinegar and pepper in’t’ but Sir Toby believes ‘it comes from a clodpole’. Olivia bids farewell to 'Cesario', asking 'him' to ‘come again tomorrow’ and Sir Toby then tells 'Cesario' that a knight with ‘youth, strength, skill and wrath’ is waiting to fight with him. Sir Toby and Fabian go between 'Cesario' and Sir Andrew telling each of them lies about the other.

Antonio arrives just as ‘Cesario’ and Sir Andrew face each other to fight. Antonio tries to stop them, thinking he is defending Sebastian. Just then, officers arrive to arrest Antonio for his past crimes against Orsino. Antonio asks ‘Cesario’ for help but 'Cesario' says 'he' doesn’t know him.

Sebastian is confronted, first by Feste and then by Sir Andrew, Sir Toby and Fabian, who all mistake him for ‘Cesario’. Sir Andrew strikes him and Sebastian strikes back, thinking everyone is mad. He almost gets into a sword fight with Sir Toby but Olivia interrupts them. She sends Sir Toby and his friends away and invites Sebastian to her house, thinking he’s ‘Cesario’. Sebastian happily follows Olivia.

Keeping up the pretence that Malvolio has gone mad and is possessed by devils, Maria and Sir Toby have locked Malvolio in a dark room. They get Feste to visit him, first in disguise as a priest called Sir Topas and then as himself.

Sebastian wonders whether he is mad or Olivia is mad but decides he likes her. Olivia asks him to go ‘with this holy man / Into the chantry by’ and there promise to marry her. Sebastian willingly agrees.

'Ceasrio' is going with Orsino on a visit to Olivia and Antonio is brought in by officers. Orsino recognises Antonio as a ‘Notable pirate’ but 'Ceasrio' recognises him as the man who ‘did me kindness’, stepping into help 'him' against Sir Andrew. Antonio still thinks 'Ceasrio' is Sebastian and explains how he saved the boy’s life three months ago but Orsino says that is ‘madness’.

When Olivia arrives, Orsino becomes angry that she is still not interested in him and is more interested in his servant. Orsino threatens ‘Cesario’ out of jealousy but 'Cesario' says 'he' would willingly die ‘a thousand deaths’ for Orsino. When Olivia hears this she is really shocked, thinking that 'Cesario' has only just married her. The priest is called and confirms their ‘interchangement of rings’ happened only two hours before.

Sir Andrew and Sir Toby interrupt. They are both injured from fighting with a gentleman they think is Cesario. They are followed by Sebastian who comes in apologising to Olivia for hurting Sir Toby. Everyone is amazed to witness ‘One face, one voice, one habit, and two persons’ as Sebastian and 'Ceasrio' face each other. The truth comes out at last that both twins were saved from drowning and that 'Ceasrio' is really Viola and a woman.

Feste gives Olivia a letter from Malvolio and she sends Fabian to release him. When he arrives, Malvolio shows Olivia the letter written by Maria and Olivia realises it is a forgery. Fabian tells her that ‘Maria writ the letter at Sir Toby’s great importance’ and that Sir Toby and Maria are now married. Malvolio walks off declaring revenge ‘on the whole pack of you’. Olivia and Sebastian, Orsino and Viola look forward to their joint wedding celebrations.