Act 1
Scene 1: Count Orsino is love-sick for his neighbour Olivia, who rejects him on a pretence of mourning her dead brother.
Scene 2: Viola arrives in Illyria, shipwrecked and believing her twin brother has died; she will take disguise as a man and serve Orsino.
Scene 3: Sir Toby Belch, Olivia's drunken relative, and Maria her maid, are exploiting foolish Sir Andrew Aguecheek.
Scene 4: Viola, become Orsino's servant Cesario, is charged to woo Olivia but admits to us she would rather be Orsino's wife herself.
Scene 5: Humorous dialogue: Maria and Feste the Clown; Olivia, Malvolio, and Clown; Maria, then Sir Toby, then Malvolio, leading to introduction of Cesario; dialogue between Olivia and Cesario where Olivia falls in love with "him;" Olivia sends Malvolio after Cesario with a ring.

Act II
Scene 1: Viola's twin brother, Sebastian, enters, having also survived the shipwreck, with Antonio his friend; they set out for Orsino's court though Antonio has enemies there.
Scene 2: The haughty steward Malvolio gives Cesario the ring from Olivia: "I left no ring with her" "she loves me sure" "Time, thou must untangle this."
Scene 3: Sir Toby and Sir Andrew, drunk and boisterous. Clown sings, Maria comes; Malvolio scolds them all, they mock him, he gets angry; they plan a revenge trick.
Scene 4: Cesario and Orsino talk of love, Cesario with hidden meanings; Clown's song; further allusive talk "My father had a daughter loved a man."
Scene 5: The garden scene, in which Malvolio finds and reads a letter he thinks sent to him by Olivia, but in fact written by Maria, while Sir Toby, Sir Andrew, and Fabian, watch from behind a hedge.

Act III
Scene 1: Comic dialogue between Cesario and Clown; Sir Andrew notes difference between his own style and Cesario's; Olivia tells Cesario of her feelings, provoking a dialogue on the theme of illusion and reality.
Scene 2: Sir Toby plans a joke duel between Sir Andrew and Cesario; Maria announces that Malvolio has obeyed the letter, and is smiling, dressed in yellow stockings with tight garters.
Scene 3: Sebastian and Antonio arrive, Antonio lends Sebastian his purse.
Scene 4: Maria prepares the encounter between Malvolio and Olivia; the dialogue between them; Malvolio alone. Sir Toby and Maria say he is possessed. Sir Andrew writes a challenge to Cesario; they are brought together, both terrified of the other; Antonio intervenes, is arrested, asks for purse; Cesario cannot understand what he is talking about, wonders if Sebastian is alive.

Act IV
Scene 1: Same misunderstanding in reverse, the Clown takes Sebastian for Cesario, brings him to Olivia; Sir Andrew thinks this is Cesario, hits him and really gets beaten; Olivia takes the bewildered but not-unwilling Sebastian home with her.
Scene 2: Malvolio is mocked by the Clown, before he brings paper for him to write a letter to Olivia.
Scene 3: Olivia prepares to be betrothed to Sebastian.

Act V
Scene 1: Clown refuses to show Fabian the letter; Orsino comes with Cesario and Antonio confronts them, tells his story both past and present, accuses Cesario; explanations are prevented by the arrival of Olivia, who rejects Orsino without a hearing. Orsino threatens to kill her, or Cesario whom he suspects. Cesario is willing to die at his hands. Olivia reveals her betrothal, which is confirmed by the priest; Sir Andrew comes in claiming to have been wounded by Cesario, but finds him here. Sir Toby arrives, also beaten. Sebastian enters, not seeing Viola, and addresses Olivia; mutual recognition, explanations. Duke asks Viola for her hand; Clown reads the letter, Malvolio is freed, the trick is revealed: "I'll be reveng'd on the whole pack of you!" Duke sends after him to entreat him to a peace. Feste sings "When that I was and a little tiny boy."

In writing Twelfth Night, Shakespeare was looking again at the play by Plautus, Menaechmi, in which separated twin brothers are mistaken for each other, that he had adapted as his Comedy of Errors. The main plot of this play, though, came from the tale of Apolonius and Silla in a collection by Barnabe Riche, which translated from the French of Belleforest a story by the Italian Bandello which was based on a play, Gl'Ingannati (The Deceived), acted in Siena in 1531 that Shakespeare may have known directly. Here the theme of a girl disguised as a boy being obliged to carry to another woman the love messages of a man she loves is central. Disguise and identity are the theme of this, perhaps the most warm-hearted of all Shakespeare's plays.

The only shadow on the sense of festivity comes from the kill-joy hypocrite Malvolio, who should not be seen as a sincere Puritan. It seems very likely that he is intended to be the parody of a particular enemy of Shakespeare's company in a quarrel they were having, the so-called Poet's Quarrel against Ben Jonson which John Marston had recently launched with a play called What You Will (the subtitle given to Twelfth Night, the only play to have such a subtitle). Malvolio may well have been recognized as Ben Jonson by people who knew him, especially in his social climbing, his hope of wooing a noble lady, his lack of any sense of humour, and his hostility towards his social equals. In modern productions, Malvolio has sometimes been acted with so much sympathy that it is the Clown, Sir Toby, or Maria who seem to be unkind! Yet the scene when he reads the letter they have planted, while they watch and make comments from behind the hedge, is probably the funniest in any of the plays.

One of the striking things in this play is the number of songs given to the clown; this is why Robert Armin has been credited with a fine voice! The songs often have little in common with the moment in which they come; one of the finest love songs is sung to Sir Toby and Sir Andrew when they are very drunk:

O mistress mine, where are you roaming?
O stay and hear, your true love's coming,
That can sing both high and low.
Trip no further, pretty sweeting:
Journeys end in lovers meeting,
Every wise man's son doth know.

What is love? 'Tis not hereafter,
Present mirth hath present laughter:
What's to come is still unsure
In delay there lies no plenty,
Then come kiss me, sweet and twenty:
Youth's a stuff will not endure. (Act II.iii)