Viola and her twin brother Sebastian, while on a sea voyage, are shipwrecked. Viola is rescued and Sebastian is supposed to have been lost. Among the wreckage on the coast Sebastian's trunk is found by Viola. When she opens it and sees her brother's clothing, she decides to disguise herself as a boy. Learning that she is within the realm of the young Duke Orsino, she repairs to his castle, where she is employed as his page. The Duke is apparently very much in love with Olivia, a rich Countess. Nothing daunted, Viola falls in love with the Duke, who adds to her discomfiture by sending her, as his page, with a message of love to Olivia. As soon as Olivia beholds the handsome young page, she falls in love with Viola, thinking that she is a boy. She presents the page with a jewel, and later, through her pompous steward. Malvolio, with a ring. Sebastian, in search of his sister, meets Viola by chance at the house of Olivia and the two are once more united. Explanations follow, and Olivia finds it an easy matter to transfer her affections to Sebastian, who falls madly in love with Olivia. Orsino finds Olivia in the embrace of Sebastian. Viola now appears before the Duke as herself, a sweet and attractive girl. She tells him of her impersonation of her brother and the page. Orsino is so struck by her beauty and cleverness that he declares his love for her and asks her to become his wife. Each one comes into his own. "All's well that ends well" and everybody is happy. A delicate touch of comedy is introduced in this play in the scene where Maria forces an affectionate note to Malvolio, imitating Olivia's handwriting and leading the arrogant steward to believe the countess in love with him. Malvolio is so puffed up and elated when he gets the note that he cannot contain himself, much to the amusement of Sir Toby. Maria and Sir Andrew, who are watching him and greatly enjoying the stroke.
—Moving Picture World synopsis