Viola is introduced to us as a survivor of a shipwreck. Her brother was lost at sea but may not be dead. Viola learns from the Sea Captain of their doomed ship that they are now in Illyria, which is ruled by Orsino. The Sea Captain explains to Viola that The Duke of Illyria is pursuing the fair Olivia, a woman who like Viola has lost a brother.
Identifying with Olivia's grief, Viola wishes to serve Olivia but when she learns this will be impossible, Viola instead has the Sea Captain disguise her as a boy so she can serve Orsino, The Duke of Illyria.