They are just about to fight when Antonio shows up and asks to defend Cesario, whom he mistakes for Sebastian. When Sir Andrew starts to notice that Olivia is in love with Cesario, Sir Toby encourages him to challenge Cesario to a duel. Antonio has grown so attached to Sebastian that he follows him into Orsino's territories despite the fact that Orsino is an old enemy. Viola's twin brother Sebastian soon arrives up in Illyria: he was saved from the shipwreck by a local, Antonio, but thinks his sister is dead. I.' Following 'Olivia's' instructions, he behaves so oddly that she worries that he has gone mad. Malvolio finds the letter and assumes that he himself must be 'M. I.' the letter instructs its anonymous addressee to wear yellow stockings and crossed garters, to act haughty, smile constantly, and refuse to explain himself in order to show that he returns Olivia's affections. She forges a letter, supposedly from Olivia, addressed to a secret beloved 'M. Maria devises a prank to get even with Malvolio. While the plot between Orsino, Viola, and Olivia unfolds, scenes at Olivia's house introduce a second group of characters: Olivia's uncle, the drunkard Sir Toby his equally vulgar friend and suitor of Olivia, Sir Andrew Aguecheek Olivia's charming lady-in-waiting, Maria the clown, Feste and Malvolio, Olivia's self-important steward, who constantly scolds and irritates the rest of them. At the same time, when Orsino sends Cesario to Olivia's house to woo her in his stead, Olivia becomes passionately attracted to 'Cesario.' Only Viola understands the love-triangle that her disguise has brought about: she loves Orsino, Orsino loves Olivia, and Olivia loves her. However, she soon finds herself falling in love with Orsino-a love which she cannot pursue, since Orsino believes her to be (the male) Cesario. Viola advances quickly in Orsino's household. Viola resolves to conceal her identity-she dresses up as a pageboy, Cesario, and go work for Orsino. However, after the Captain informs Viola that Olivia refuses to see anyone. Grieving, she learns from the Captain-who, by chance, was born in Illyria-about the region, and decides that she would like to conceal her identity and offer her services to this Lady.

Finding herself alone with the Captain, Viola assumes that her twin brother, Sebastian, with whom she was traveling, is dead.

Meanwhile, a ship has been wrecked by a storm off the coast, casting the young noblewoman Viola onto shore. In the kingdom of Illyria, the Duke Orsino laments over his unrequited love for the Lady Olivia, who is in mourning for her brother and has refused to see anyone for seven years.

Twelfth night by william shakespeare