Ummm... there's lots of people in love in strange directions, and at the end, noone marries the person they fell in love, but they all marry people they fall in love with later, and everybody is happy except the guy who was locked up for wearing funny socks. There's your plot summary.
Otherwise, the whole scene where Malvolio dresses up in yellow and cross garters and smiles as wide as possible in order to seduce Olivia was right hilarious, no question. Honestly, I kind of got the feeling that the rest of the story was an excuse for the Malvolio stuff. :P. But, I DID learn where the whole line about "Some men are born great" etc came from, and that it was actually sarcastic, here, which was a charming little revelation. All in all, not my favorite play. But it could be fun if staged well.
Funny socks, you say? So, um, why was that a criminal offense?
Actually, locked up in an asylum, not in prison.
Well, okay, but why? Wearing funny socks was an indicator of insanity?
OK... he's the steward under Viola, a noble Lady, and a priggish bore, so one of the ladymaids plays a trick on him. She writes a letter in the lady's handwriting that intimates that she is in love with him, and telling him, if he should find the letter, he should wear yellow socks and cross garters, and smile all the time, and be insolent to people, because these are the things she loves. So he traipses around in bright socks talking about his garters with a big goofy smile plastered on his face. So, everyone decides he's gone mad, because he keeps saying weird things about his garters and what not, and treats his betters as if they were servants, and they lock him up.