1. Orsino declares his love for Olivia (Act 1, Scene 1) 

Orsino, Duke of Illyria, declares his love for his neighbour, Countess Olivia, but his messenger tells him that she is in mourning for her dead brother and will not see any visitors for seven years.

2. A shipwreck (Act 1, Scene 2) 

A shipwreck strands Viola on the shores of Illyria and she believes that her identical twin brother, Sebastian, is drowned. She persuades the captain of the ship to dress her as a man and present her as a servant to Orsino.

3. Disorder in Olivia's household (Act 1, Scene 3) 

Sir Toby Belch, Olivia's uncle, is criticised by the maid, Maria, for being drunk and upsetting the peace of his mourning niece but he takes no notice. Sir Andrew Aguecheek, a foolish knight, is persuaded to stay another month in the hope of persuading Olivia to marry him. He needs her money.

4. Viola is employed by Orsino (Act 1, Scene 4) 

Viola, disguised as a man and using the name Cesario, is now working for Orsino. S/he is sent with a message of love from him to Olivia but reveals to the audience that she has fallen in love with Orsino herself.

5. Olivia and Cesario meet for the first time (Act 1, Scene 5) 

Feste, Olivia's jester, teases his mistress out of her sadness but Malvolio, her steward, does not find him funny. Viola (dressed as Cesario) meets Olivia and declares Orsino's love for her. Olivia immediately falls in love with the young 'man' and asks 'him' to come again. To make sure of a second visit, she sends Malvolio after Cesario with a ring, saying it is a gift from Orsino that she wishes to return. This confuses Cesario, who fears that Olivia has indeed fallen for her because she is dressed as a man.

6. Malvolio and Sir Toby cross swords (Act 2, Scene 3) 

Sir Toby, Feste and Sir Andrew get very drunk and create a loud scene in the middle of the night. Malvolio comes down to remonstrate with them, but to no effect. Maria suggests a way in which the drinkers can get their own back.

7. Malvolio's downfall is plotted (Act 2, Scene 5) 

Hurt by Malvolio's earlier contempt and fearing that it might cost him his job, Feste plots with Sir Toby and Maria to disgrace Malvolio in front of Olivia. It is agreed that Maria will write a love letter to Malvolio which seems to come from Olivia and leave it where he cannot miss it. In it, he is asked to wear yellow stockings, cross-gartered and to smile – three things which Olivia hates. They hide behind a hedge to see the effect of the letter. Malvolio is convinced that Olivia loves him and is determined to follow the letter's instructions.

8. Another shipwreck survivor arrives in Illyria (Act 3, Scene 3) 

Viola's brother, Sebastian, arrives in Illyria, believing that his twin sister is drowned. He is accompanied by a loving sailor, Antonio, who is risking his life because he is a wanted man in Illyria.

9. Malvolio is imprisoned, Sir Andrew issues a challenge and Viola learns that her brother may still be alive (Act 3, Scene 4) 

Following the letter's instructions, Malvolio visits Olivia in yellow stockings and cross-gartered. She takes his smiling as a sign of madness and asks Sir Toby to take care of him. Toby, Maria and Feste imprison him in a dark dungeon.

Sir Andrew, convinced that Olivia favours Cesario over him, is persuaded by Sir Toby to challenge 'him' to a duel. Both equally terrified, Andrew and 'Cesario' fight but are interrupted by Antonio, who takes up the fight believing that 'Cesario' is his friend, Sebastian. Antonio is then arrested and begs help of 'Sebastian', who appears to reject him.

10. Olivia believes she has found happiness (Act 4, Scene 3) 

Olivia meets Sebastian and, believing him to be Cesario, is delighted when he agrees to marry her immediately.

11. The resolution (Act 5, Scene 1) 

Orsino and 'Cesario' meet Antonio and the guards on their way to prison, and are confused by his belief that 'Cesario' has abandoned him. Olivia arrives and announces that she and 'Cesario' are newly married. Orsino believes her and turns on 'Cesario', who pleads that s/he loves him.

When Sebastian appears, the twins are re-united, the confusion is resolved and Orsino and Viola are free to proclaim their love for one another.

The trick played on Malvolio is revealed and we learn that Sir Toby has married Maria. Malvolio is released from his prison cell and swears revenge on them all.