Play Summary (Courtesy of Play Shakespeare)

Duke Orsino of Illyria is in love with Olivia, but his advances are rejected. A shipwrecked Viola arrives on his shores, and with the help of a Captain, disguises herself as a boy, calling herself Cesario, and enters Orsino’s service. Orsino takes to Cesario, and sends ‘him’ to woo Olivia for him. Viola, however, is already falling in love with Orsino.

Cesario arrives to woo Olivia, and Olivia falls in love with "him”. Olivia rejects Orsino’s approach, but asks Cesario to return. Orsino discusses the nature of love with Cesario, and sends “him” again to Olivia, who continues to confess her love for Cesario, much to “his” dismay. Meanwhile, Viola’s twin brother Sebastian, also a casualty of the shipwreck, arrives in Illyria with the help of his close friend Antonio, a sea-captain and former enemy of Orsino. Antonio gives Sebastian his purse full of money to use, and tells him to go to an inn where they will meet up later.

Staying with Olivia is her uncle Sir Toby, who is encouraging Sir Andrew, his drinking-companion and source of funds, to woo Olivia. While carousing with Olivia’s fool Feste late one night, they quarrel with Malvolio, and with the help of Olivia’s maid Maria, they decide to trick him. Maria writes a letter to Malvolio, forging Olivia’s handwriting, to make Malvolio think Olivia loves him. The letter asks Malvolio to dress and behave in eccentric ways.

Toby, Andrew, and Maria (as Fabian) observe Malvolio opening the letter and absorbing its contents. Malvolio then approaches Olivia according to the letter’s instructions, and she thinks him mad. Toby arranges for him to be confined in a dark room. Later, he gets Feste to disguise himself as a priest, Sir Topas, to taunt Malvolio. Malvolio writes a letter of complaint to Olivia.

Meanwhile, Andrew prepares to leave the house, having seen Cesario apparently being more successful with Olivia, but Toby persuades him to stay, and to challenge Cesario to a duel. Sir Toby separately informs Andrew and Cesario that the other is a ferocious fighter, and they approach each other with trepidation. They are about to fight when Antonio arrives, intending to defend Cesario, whom he has mistaken for Sebastian. Antonio is arrested by officers who recognize him as Orsino’s enemy. He asks for his purse from Cesario, who of course professes no knowledge of it.    

The fool Feste is sent to bring Cesario to Olivia, but encounters Viola/Cesario’s twin brother Sebastian instead. They meet Andrew, who strikes Sebastian mistaking him for his twin Viola/Cesario. Olivia arrives to stop a fight between Sebastian and Toby. Sebastian is immediately taken with Olivia. Thinking him to be his twin Cesario, Olivia is delighted that he has finally responded to her affections, and they go off to be married.

Orsino is told by Cesario of his enemy Antonio’s arrival, but when Antonio is brought before Orsino, he mistakes Cesario for twin Sebastian, his dear friend, and asserts that he has been with Cesario for some time. Much confusion happens due to mistaken identity, not knowing there is a set of twins, and mistaking Viola/Cesario for Sebastian and Sebastian for Viola/Cesario! 

Olivia arrives, and again rejects Orsino’s advances. As Orsino and Cesario prepare to leave, Olivia insists on Cesario staying, and calls him her husband. When Cesario denies it, she presents the priest as confirmation. Orsino is enraged, thinking Cesario has betrayed him. Toby and Andrew pass by, having just been severely beaten by Sebastian. Sebastian then arrives, and the twins delightedly recognize each other. Everyone is amazed, Viola’s true identity is revealed, and she and Orsino decide to be married.

Feste delivers Malvolio’s letter to Olivia, and he is brought from his cell. Maria/Fabian reveals the nature of the trick played upon him, and Malvolio leaves, vowing revenge on them all. They all prepare for celebration, leaving Feste to bid the audience farewell.

Setting

While Illyria existed in antiquity, at the time of Shakespeare’s penning of Twelfth Night the region was a mythic place many of his contemporaries also used as a fictional setting. Located off the Adriatic Coast in territory that now includes parts of Croatia, Serbia, and Bosnia, as well as other regions, Illyria ceased to exist as an independent kingdom when it was conquered by the Romans in the 2nd Century BCE, and was reorganized as a Roman province called Illyrium, which was subsequently re-divided and renamed in 10 BCE.

Viola and Sebastian’s home country of Messaline is even more of an invention by Shakespeare. Some scholars have suggested that Messaline is meant to correspond to either Mytilene, in Greece (on the Isle of Lesbos) or Messina, in Italy. The name may also come from Valeria Messalina, the third wife of Roman emperor Claudius. She was a paternal cousin of Emperor Nero, a second cousin of Emperor Caligula, and a great-grandniece of Emperor Augustus. A powerful and influential woman with a reputation for promiscuity, she allegedly conspired against her husband and was executed on the discovery of the plot.