Twelfth Night by William Shakespeare

Viola is shipwrecked on the coast of Illyria and comes ashore with a captain's help. She has lost contact with her twin brother, Sebastian, who she believes has drowned. With the aid of the Captain she disguises herself as a young man named Cesario and enters the service of Duke Orsino. Orsino has convinced himself he is in love with Olivia who is in mourning and who therefore refuses to see entertainments, be in the company of men, or accept love or marriage proposals from anyone, including Orsino, until seven years have passed. Orsino then uses 'Cesario' as an intermediary to profess his passionate love for Olivia. But Olivia falls in love with 'Cesario', setting her at odds with her professed duty. Meanwhile, Viola has fallen in love with Orsino, creating a love triangle: Viola loves Orsino, Orsino loves Olivia, and Olivia loves Viola disguised as Cesario.

Meanwhile, Viola's twin, Sebastian, has been rescued by Antonio, a sea captain who previously fought Orsino, yet who accompanies Sebastian to Illyria, despite the danger, because of his admiration for Sebastian.

With their love for practical jokes, Sir Toby and Fabian convince Sir Andrew to challenge Cesario to a duel, knowing that neither of them can fight. Their initial duel is interrupted by Antonio, who believes Cesario to be Sebastian. Orsino's officers then arrest Antonio. Emboldened by this, Sir Andrew mistakes Sebastian for Cesario and slaps him, prompting Sebastian to beat up Sir Andrew. Olivia witnesses the skirmish and chastises Sir Toby, Sir Andrew, and Fabian. Taking Sebastian for 'Cesario', Olivia asks him to marry her, and they are secretly married in a church. Finally, when 'Cesario' and Sebastian appear in the presence of both Olivia and Orsino, there is more wonder and confusion at their physical similarity. At this point, Viola reveals her identity and is reunited with her brother.