Viola was washed ashore only to find that her brother was no where to be found. A helpful sea captain helps disguise herself as a man and get work for the Duke Orsino.
Viola now disguised as Cesario, goes to Orsino for a job. She is to be his assistant, but, while she works for him she starts to fall in love even though she is a 'man' so she can't say anything. She the gets asked to go deliver a love message to Olivia and finds out that she is the one he loves
Cesario delivers the message but instead of Olivia falling in love for Orsino, she falls in love with Cesario. The love triangle is then complete, Viola loves Orsino- Orsino loves Olivia- Olivia loves Cesario.
YOU A TRAITER SEBASTIAN. A TRAITER!!
Sir Andrew, jealous of Cesario (still Viola in disguise), challenges Cesario to a duel, but Sebastian comes onto the seen and since he looks so alike to Cesario (Viola in disguise) they mistake him for Cesario and have a duel with Sebastien instead.
In the middle of the duel Olivia comes over and proposes to him, while he is confused because he has never seen her before, he sees that she is rich and accepts. Now Olivia has married Sebastion thinking he is Cesario.
Antonio is getting arrested while Sebastian is with Olivia and cries out to Cesario, mistaking him for Sebastian, asking him to help him. Cesario is confused and says nothing. Then Cesario realises that her brother could be alive.