First of all, the original version of Twelfth Night is a work by Shakespeare.
It happened in Illyria. A pair of twin brothers, Viola and Sebastian, drifted here due to a shipwreck. However, neither of them could find each other. They believed that the other had perished. In order to survive, Viola disguised herself as a man and worked as a valet for the local ruler, Duke Orsino. Therefore, she fell in love with the duke. Before that, the duke had He had long fallen in love with Countess Olivia, so the Duke sent Viola to woo Olivia on his behalf, who had repeatedly failed to woo him. As a result, Olivia fell in love with Viola, who was disguised as a man. Later, she got engaged to Viola's brother because of a misunderstanding. In the end, the Duke and Viola, Olivia and Sebastian were together, ending in a comedy.
That's about it. This is the introduction made by Nian Mian himself. I didn't ask Du Niang. I feel that the above one is a bit too brief. Is this okay?