Viola thinks that Sebastian is dead due to a shipwreck. She talks to the Captain to confirm this. She ends up using this to her advantage.
Viola dresses up as a man and calls herself Cesario to get closer to Orsino, the king because she loves him, and she wants to work for him.
Cesario (viola) soon finds out that Orsino is in love with lady Olivia. Orsino sends cesario to tell Olivia about Orsinos love to Olivia, but Olivia falls in love with Cesario.
Maria,Sir Toby and Sir Andrew played a trick on Malvolio. They wrote a letter to Malvolio in Olivia's handwriting to trick him into thinking she loves him. He ends up doing all these things to get her attention but he ends up looking crazy and gets locked up.
Sebastian survives and turns up in illyria where cesario is. they both start doing things and it get everyone confused. Olivia confuses cesario for Sebastian and Sebastian marries her causing conflict for viola. (cesario)
Viola is revealed and she reunites with her brother Sebastian. Olivia and Sebastian remain in love and Orsino becomes a couple with Viola. Malvolio gets reality check when Olivia reveals ht the love letter to him was fake. Everyone is happy in the end with their significant other.