What are the plot and subplot in Twelfth Night and how are they interlinked?

In the main plot, Duke Orsino uses the shipwrecked Viola, who is disguised as a man named Cesario, to help him woo the Countess Olivia. However, comic mayhem follows as Olivia instead falls in love with Cesario, not realizing, of course, that "he" is really a woman. That wasn't supposed to happen. Meanwhile Cesario/Viola falls in love with the Duke. So a woman falls in love with a woman disguised as a man, and a woman disguised as man falls in love with a man. It doesn't get much more gender bending than this. As Shakespeare said in another play, the course of true love never did run smooth though, this being a comedy, everything sorts itself out in the end. 

The subplot involves servants. Olivia's servant Maria, called her "woman," wants revenge on Olivia's steward Malvolio, who has an inflated ego. Maria plays on this weakness, dropping love letters to him that are supposedly by Olivia. A steward with a more realistic self image would realize that a countess wouldn't fall in love with him, but the foolish and self-infatuated Malvolio falls for the scheme. 

As is often the case in Shakespeare, the main plot involves highborn or aristocratic characters and the subplot lower-class characters. The subplot is linked to the main plot through characters such as Olivia but, more importantly, through the common themes of love, lunacy, and deception. Malvolio may be the one jailed for lunacy but, Shakespeare asks, isn't any form of love a kind of self-deceiving madness?

Discuss the plot and subplot in Twelfth Night.

Of course, the best way for you to find out the answer to your question is to read or see the play. There are some very good film editions available as well which can also help. But remember that these are no subtitute to good, rigorous textual analysis. Rather than give you a complete breakdown of the play, I am going to talk you through one of the chief features of the play - the central confusion of love between three characters in the main plot - which you can then go back to and "flesh out" with your own reading.

Typically as one of Shakespeare's comedies, this play involves mistaken identities, cross-dressing, disguises and people falling in love with people they shouldn't fall in love with. Thus it is that we are introduced in Act I scene i to the Duke Orsino, who is swift to declare his undying love for Lady Olivia:

O, when mine eyes did see Olivia first,

Methought she purged the air of pestilence.

That instant was I turned into a hart,

And my desires, like fell and cruel hounds,

E'er since pursued me.

This situation is immediately complicated by news of Olivia's unremitting resolution to not marry for love of her dead brother:

The element itself, till seven years' heat,

Shall not behold her face at ample view,

But like a cloistress she will veiled walk...

So, it is clear that Olivia does not return these feelings, as we see from her distaste of being forced to listen to them once more from Cesario. However, this situation is further complicated in Act I scene iv by the irony of Viola dressed as Cesario being forced to take messages of love from her master to Olivia when she is in love with Orsino herself!

Yet a barful strife!

Whoe'er I woo, myself would be his wife.

Then of course note Olivia's response to Cesario in Act I scene v:

Even so quickly may one catch the plague?

Methinks I feel this youth's perfections

With an invisible and subtle stealth

To creep in at mine eyes.

So, three characters, each involved in a crazy tangled up relationship involving disguises, secret loves and unrequited love. The real question of course is how on earth is Shakespeare going to resolve this situation in this "comedy". Now, as you read through the play, make sure you pay attention to how the subplot relates to the overall plot - how do characters such as Sir Toby relate to the confusion in the main plot? What about the relationship between Sir Toby and Maria? This will help you think through how the subplot and plot relate in this great play.