In Illyria, Duke Orsino is in love with Olivia, a noblewoman who has recently lost her brother. He sends his servant Valentine to speak with her, but Valentine returns with a refusal. Olivia will not receive visitors. She says she will mourn her brother for seven years. Orsino admires her devotion and thinks that if she can love her brother so deeply, she will be capable of loving him too.

Meanwhile, Viola has survived a shipwreck. She is afraid that her brother has drowned, but the captain who rescued her says he saw him tie himself to a strong mast and stay afloat. Viola gives the captain money for the hopeful news. She asks him to keep her secret and help her disguise herself as a young man. She plans to serve Orsino, whose name she knows from her father. The captain agrees to help.

At Olivia's house, her uncle Sir Toby Belch is drinking and making noise. Maria, Olivia's waiting-woman, tells him that Olivia dislikes his late-night behavior. Sir Andrew Aguecheek, a rich but foolish guest, is trying to win Olivia. Maria thinks he is a spendthrift and a coward, but Sir Toby encourages him to stay. Sir Andrew is good at dancing and languages, yet he often misunderstands people.

Viola, now called Cesario, has become a trusted servant in Orsino's household. After only three days, Orsino sends her to speak with Olivia on his behalf. He thinks her smooth face and high voice will make her a good messenger. Viola agrees to go, but she secretly wishes that she could be the one loved and married, instead of carrying someone else's message.

At Olivia's house, the fool Feste answers questions with jokes and puzzles. Olivia asks why he mourns a brother whose soul is in heaven. Malvolio, another servant, complains that the fool is not amusing. Olivia defends him. Maria reports that a young man is waiting at the gate. Sir Toby is too drunk to help. Malvolio says the visitor will not leave, no matter how many times he is refused.

Olivia lets the young man in. Viola does not know which woman is Olivia, so she asks. Olivia answers for herself. Viola begins to deliver Orsino's message, but Olivia interrupts the long praise and asks for the important part. Viola tells her that Orsino loves her. Olivia says she cannot love him. She knows he is noble, rich, young, and well spoken of, but that is not enough.

Viola imagines loving someone so much that she would not understand being refused. She says she would keep calling the person's name and asking for pity. Olivia asks about her family. Viola says she is a gentleman, though her fortune is not as high as her birth. Olivia offers payment for the message, but Viola refuses. She wants Orsino, not herself, to be rewarded.

After Viola leaves, Olivia feels drawn to her. She sends Malvolio after the young messenger with a ring, saying she does not want Orsino's gift. She also invites him to come again if he wants to hear her reasons. Olivia is surprised by her own feelings and wonders whether her eyes are flattering her. She leaves the matter to fate.