What part do the witches play in the plot of Macbeth?What part do the witches play in the plot and how is it represented in the script?

In Shakespeare's Macbeth, the witches
serve as the catalyst for the plot.  They ignite Macbeth's dormant
ambition. 

The witches, at the least, appear to be
supernatural.  They appear to know the future, though they don't appear to control it. 
When they give Macbeth a bit of truth in order to deceive him and lead him to ruin, as
Banquo predicts, Macbeth takes the idea that he will be king, and turns that into he
will be king now, as does his wife.  The play reveals Macbeth as a
loyal thane to Duncan, until the witches suggest the idea of his being king.  Though
slight evidence exists that Macbeth has thought of being king before, and of what his
being king would mean (he'd have to kill Duncan to do it), all evidence suggests that
that ambition would have stayed inactive, except for the
witches. 

Concerning plot, that is the central contribution
of the witches.  Of course, when Macbeth seeks them out in Act 4.1 they crystalize their
contribution while elaborating on their original predictions, which leads to further
acts of violence by Macbeth, such as the ordering of the deaths of Macduff's
family. 

The second meeting with the witches, by the way,
also provides another chance for Shakespeare to gain favor with King James I, his
patron.  The parade of kings descended from Banquo would have included King James, since
he was an actual descendant of Banquo.