Viola and a sea captain and several sailors enter. They have been shipwrecked on the seacoast of Illyria and have barely escaped drowning. The captain congratulates Viola on not being drowned, for he tells her that when their ship split in half, he saw her brother, Sebastian, tie himself to a mast; yet even that, he fears, did not save Sebastian, for he saw him and the mast borne away on the waves. According to the captain, there is a slim chance that Sebastian survived, but there is a strong possibility that only the captain, Viola, and these few sailors are the sole survivors. Viola is appreciative of the captain's kind, if cautious, optimism; she gives him some gold coins and asks him if he has any idea where they are. The captain does; he knows Illyria well. He was born and reared here, and he tells Viola that the country is governed by a "noble Duke," Duke Orsino. Viola recognizes the name; her father spoke of him. The duke is a bachelor, she believes.
The captain is not so sure that this fact is still true; he says that according to current gossip, the duke has been seeking the love of "fair Olivia," but he says that Olivia is a virgin and that she is determined to remain so. Following the death of Olivia's father (a year ago) and the death of her brother (just recently), Olivia forswore men altogether.
The story intrigues Viola; she herself is now in mourning for her brother, Sebastian, and nothing would please her more than to serve Olivia. The captain, however, says that such a plan is impossible. Olivia will see no one. For a moment, Viola ponders, then she devises an ingenious scheme. She will disguise herself as a young eunuch, and she will pay the captain handsomely for his aid if he presents her to Duke Orsino. She will sing for the duke, play any number of musical instruments for him and — in short — she will ingratiate herself in his household. The captain agrees, and they exit.
With the shift of this scene to the seacoast of Illyria, we meet another principal character in the comedy — Viola — and in meeting her, we hear more about the Lady Olivia, and even though their names are almost perfect anagrams (a rearrangement of the same letters in the names), and even though they are in similar dramatic situations in this play, they are vastly different women. Both of them have recently been orphaned, and, to all outward semblances, both have lost a brother and are therefore alone in the world. But here the similarity ends. Olivia is indulging in her grief, but whereas Viola deeply grieves for her brother, she is still able to function in the practical world. Unlike Olivia, Viola, shipwrecked and alone, does not have time to indulge in her grief. Being a shipwrecked virgin maid on a strange shore and knowing no one, she must use her wit, her intelligence, and her ability to analyze situations and characters. Consequently, Viola decides to disguise herself as a man for a very practical purpose — to assure her own protection in an alien world which would not respect a young virgin maiden. And with the assumption of this disguise, we will have the beginning of a complicated series of disguises which will run throughout the remainder of the comedy.
Viola's uncanny ability to intuit other people's ideas enables her to trust the sea captain; he can help her carry out her plans and keep her identity secret. Without his trust, her plans would fail, and after she has assumed her disguise, she uses it to its fullest potential — that is, she never passes up the opportunity to use her disguise in order to make puns and double entendres for parodies and satires and, ultimately, to comment subtly on the disguised biological difference between herself and the Lady Olivia. In other words, while the disguise provides Viola with security and protection, it also allows her to utilize her wit for her own enjoyment and also for the enjoyment of the audience.