Viola was shipwrecked and rescued. She thinks her brother Sebastian drowned, so she disguises herself as a boy, Cesario. She gets a job as delivering Orsino's love letters to Olivia.
Sebastion comes to Illyria, thinking his sister is dead, with his rescuer, Antonio. Viola is in love with Orsino, who loves Olivia, who loves Cesario who is really Cesario.
Maria, Olivia's gentlewoman, sends love letters to Malvolio to make it seem like Olivia wrote them. In the letter, Maria tells Malvolioto dress and act outrageously, knowing it will annoy Olivia.
Olivia thinks Malvolio is mentally unhinged so she asks Toby to take care of him. Olivia then confesses her love for Cesario, which puts Viola in an uncomfortable position. Olivia leaves and Toby delivers the challenge of fighting Cesario to Andrew. But, Cesario (Viola) does not want to.
Olivia had mistaken Sebastion for Cesario so that brings Viola out of the uncomfortable position. Sebastion is bewildered by the professed love of Olivia and the riches she bestows him.
Sebastion recognizes his sister dressed as a boy. Olivia is happily married to Sebastion and Cesario can become Viola again. She will now marry Duke Orsino. Toby marries Maria and Malvolio gains his freedom but he plans to get revenge on Maria for tricking him. It all ends with three weddings and one happy reunion.