A twin brother and sister, Sebastian and Viola meet a storm at sea and their ship is wrecked. Both are saved, but are separated and neither knows whether the other has survived. Viola comes to a new country, Illyria. She decides to disguise herself to serve in Orsino's court. Orsino, a noble duke, is in love with a countess, Olivia, who is in her mourning to her dead brother for 7 years.
Viola, disguised as Cesario, wins the duke's trust and is sent to deliver a love message to Olivia. Ironically, Viola loves Orsino and she is on her mission to charm her rival's heart for the duke.
Before we see Viola meeting Olivia, Shakespeare introduces us a bunch of comical characters in Olivia's house: Sir Toby Belch, Olivia's fat, jolly, hard-drinking cousin, and his friend, Sir Andrew Aguecheek, a wealthy, skinny, simple-headed knight. Sir Andrew comes to woo Olivia, too. Three other comic characters are Maria, Olivia's maid; Malvolio, Olivia's steward; and Feste, Olivia's clown.
Olivia fells in love with Cesario, who is Viola in disguse, immediately. Suddenly she forgets her mourning of her brother's death. Viola is distressed when she learns of Olivia's passion for her. Meanwhile, Sebastian, Viola's twin brother, arrives in Illyria with Antonio, the man who rescued him from drowning.
Sir Toby, Sir Andrew, Feste, and Maria have a loud party at night. Malvolio comes to stop the party. The group are very annoyed and plan to revenge... They send an anonymous love letter to Malvolio. But, Malvolio thinks, the letter is from Olivia, telling him to wear yellow stockings and to smile continually at Olivia. Malvolio follows the instructions to approache Olivia, and Olivia thinks he has lost his mind. She leaves him to Sir Toby, and he imprisons Malvolio.
Sir Andrew challenges Cesario/Viola to a duel since he is mad at Olivia's crush on Cesario. Though both are anxious to avoid any real fighting, Sir Toby and Feste egg them on to the fight. Sir Andrew is a coward and Viola does not know how to fight. Then, Antonio shows up and he mistakes Viola for her twin brother, Sebastian. He fights for her. Later the officers come in to arrest him. He calls her "Sebastian" and she realizes that her twin brother must be alive.
Soon Sebastian wanders in and he is caught by Feste, Sir Toby, and Sir Andrew at another place. They mistake him as Cesario/Viola, and attack him again. Sebastian beats them up as Olivia rushes in. She reproaches her cousin for fighting and invites Sebastian into her house.
Maria and Sir Toby persuade Feste to visit the imprisoned Malvolio. Feste is disguised as a priest. He gives Malvolio paper and pencil so he can write to Olivia informing her of his situation.
Unlike Cesario, Sebastian responds warmly to Olivia's love. She arranges an immediate wedding with him in front of a priest. Sebastian who loves Olivia at the first sight gladly takes advantage of the situation.
In the Final Act, Orsino is accompanied by Cesario/Viola when he meets Olivia in person. Orsino is about to kill Viola as a sacrifice to show Olivia how much he cares about Olivia. But as Olivia calls Cesario/Viola "husband", Orsino gets angry at Cesario/Viola. Sir Andrew runs in and reports that Cesario wounds Sir Toby very bad. When everyone looks at Cesario/Viola for the answers, Sebastian appears on stage and his arrival explains all these confusions.
Malvolio is released from the prison. He says, he will take revenge. But, it seems that everyone is happy for the weddings of three couples: Orsino with Viola, Olivia with Sebastian, and Sir Toby with Maria. No one takes Malvolio's words seriously...
Twelfth Night in Productions