The play begins, like many of Shakespeare's comedies, soon after a storm in which Viola, a noblewoman, has been cast ashore. Thinking that her brother Sebastian has been lost in the storm, she determines to dress as a man so as to travel more safely in the new and hostile land. She comes under the employment of the Duke, Orsino, who is in desperate, unrequited love with the beautiful lady Olivia. The Duke sends Viola (not knowing that she is a man), to try and convince Olivia of his love.
Unfortunately, Olivia falls in love with the fine specimen of a man that Viola portrays, and asks her to return so that they might get to know each other better. To add to the confusion, Viola has secretly fallen in love with Orsino and so the three leads seem as though they will be trapped in a hopeless love triangle within which none of them can be happy.
Toby and Aguecheek lock him away in a cellar and torment him (supposedly in the hope of bringing him back to sanity). In the meantime, Viola's brother – who she thought had drowned – has arrived in the country, along with his friend Antonio. Toby and Aguecheek (who harbors romantic intentions towards Olivia himself), come across Sebastian and as they had challenged Viola to a duel, begin to fight him. This, understandably, confuses Sebastian somewhat, but he is even more confused when Olivia also arrives and (thinking he is Viola) asks him to marry her.
Completely bamboozled by this turn of events, Sebastian nevertheless thinks himself onto a good thing and accepts the beautiful stranger’s invitation. The confusion builds to a climax in which the two siblings are finally reunited, and Viola feels as though she can reveal her true sex, and her love for Orsino. The play ends in multiple marriages and happiness for everyone--apart, of course, for Malvolio, who is released from his dark hole and storms out of the house.
However the true greatness of Twelfth Night is that it is hilariously funny to watch. Whether it is the humor created by the limited viewpoint of its characters stuck in a set of increasingly bizarre circumstances, or the satire on the hypocritical puritan character who truly gets his comeuppance, an audience watching Twelfth Night cannot help breaking into waves of laughter. Constructed by a genius, and with all the hallmarks of the greatest playwright who ever wrote, Twelfth Night is a gem of a play even for those who watch it four centuries after it was first performed.