Duke Orsino is in love with Lady Olivia
Viola has been shipwrecked in Illyria, believes her brother is dead, and decides to disguise herself as a boy "Cesario"
"Cesario" enters the employment of Orsino, who sends her to court Olivia
Orsino, the Duke of Illyria, is desperately in love with the Countess Olivia, but she is mourning the death of her father and brother and will not speak with Orsino, or accept letters or messages from him.
Viola has been shipwrecked and rescued; she finds herself moored in Illyria. She mourns her the loss of her twin brother, Sebastian, who she believes drowned in the shipwreck, however, the ship's captain saw him tie himself to the ship's mast, so there is a small chance that he survived.
Viola decides that it would be wisest to disguise herself as a boy and enter the employment of Duke Orsino. She takes the name Cesario and is given the job of courting the Lady Olivia for the Duke, whom Viola has fallen in with--"Whoe'er I woo, myself would be his wife."
Sir Andrew Aguecheek, a rather ridiculous and stupid man, is another one of Olivia's suitors. He spends his time drinking with Olivia's uncle, Sir Toby Belch. Olivia pays no more attention to Sir Andrew than she does to Orsino. Olivia's steward, Malvolio, is a Puritan and as ill-tempered as his name suggests, but he desires Olivia for himself.
Cesario (Viola) shows up to court Olivia and refuses to leave. Her refusal to leave intrigues Olivia and she agrees to see Cesario. Cesario woos Olivia for Orsino, but Olivia will not listen; she is more interested in Cesario. After Cesario leaves, Olivia sends a ring that "Cesario gave her as a gift" (a purely fictional story).