The most important subplot in William Shakespeare’s play Hamlet involves Hamlet’s relationship with Ophelia. One of the earliest and fullest treatments of that relationship occurs in Act II, scene 1, where Ophelia tells her father, Polonius, about a recent and very disturbing encounter with Hamlet, as in the following brief excerpt:
Ophelia. My lord, as I was sewing in my closet,
Lord Hamlet, with his doublet all unbrac'd,
No hat upon his head, his stockings foul'd,
Ungart'red, and down-gyved to his ankle;
Pale as his shirt, his knees knocking each other,
And with a look so piteous in purport
As if he had been loosed out of hell
To speak of horrors- he comes before me.
As this short passage suggests, the subplot involving Hamlet and Ophelia is important for a number of reasons, including the following:
- It helps reinforce our curiosity about whether Hamlet is really going mad or is only pretending to be mad. Ironically, it is Ophelia who will ultimately be driven truly mad when Hamlet slays Ophelia’s father.
- It reminds us of the sort of life Hamlet might have had – happy with Ophelia – before his father’s death and before the ghost’s appearance.
- It helps contrast the goodness and innocence of Ophelia with the evil and corruption of Claudius in particular.
- It helps highlight the crueler aspects of Hamlet’s personality, especially when he speaks to Ophelia (later in the play) in ways that he knows will only upset, confuse, and torment her.
- It helps contrast the genuinely virtuous and innocent Ophelia with Gertrude, the other major female in the drama, who seems far less virtuous and innocent that Polonius’s daughter.
- It helps emphasize the loving relationship between Ophelia and her brother Laertes – the kind of uncomplicated affectionate relationship that seems rare in this play.
- It helps emphasize the irony of the fact that Hamlet, who grieves for a beloved father unjustly murdered, is himself responsible for the killing of Ophelia’s own beloved father.
- It helps emphasize the tragedy of the play when the innocent and virtuous Ophelia dies. Hamlet briefly contemplates suicide; Ophelia, apparently, actually commits it.
In short, the subplot involving Hamlet and Ophelia contributes in a multitude of ways to the richness and complexity of the play.
One subplot is Prince Fortinbras' revenge which he forgoes and ends up inheriting all of Denmark. But the big subplot is Laertes' revenge. Hamlet mistakenly kills Polonius and thus sets in motion the wheels that bring Laertes to Elsinore looking for revenge. This drives the latter portion of the play. While Hamlet is off stage for a well deserved rest, we watch Laertes with the help of Claudius plot his revenge. This and the main plot then come together in the grand finale where Laertes, Hamlet, and Claudius die.
One of the sub plots in Hamlet is the love story between Ophelia and Hamlet. In that it doesn't affect the main trajectory of the play but it is followed through the story.
The purpose I suppose would be to help build Hamlet's character, and also the sub plot inadvertently leads to Hamlet's death. When Laertes blames Hamlet for Ophelia's madness (and therefore death) and vows to take revenge.
Hope that helps.