The main character, Viola is caught in a shipwreck with her brother who she assumes is dead. She meets a Captain who is kind and tells her where she can find a job. Viola gets a job with the Duke Orsino and she's dressed as a guy.
Orsino sends her to talk about how great he is to Olivia, this girl he likes that won't marry him. Viola goes over and talks about Orsino to Olivia, Olivia falls for Viola.
While this all happens, the servants of Olivia are planning on pranking another servant, Malvolio. They give him a letter that is said to be from Olivia that says he has to act really crazy. He does so and the other servants lock him up for being crazy
Seeing that Viola doesn't love Olivia back, one of Olivia's bachelor decide to challenge Viola and during it a man comes to defend Viola and is arrested while calling Viola Sebastian. That man was Sebastian's friend who was not allowed to be in Illyria.
Sebastian, who is alive comes to the scene, gets into a fight and is proposed to by Olivia mistaking him for Viola and he says yes to it. With that Viola comes with Orsino to Olivia's place.
With some accounts of the Viola being confused for Sebastian, Sebastian comes and it's revealed that their that they are twins and they let Sebastian's friend go and Malvolio out of the cellar where he was locked up.