Wyrd Sisters is one of my top 5 DW books, it is on top of my head whenever anyone asks for a starter discword book, but they are those classic-definitioned plotholes that ruin the consistence of the plot.
Okay, let me start out and say I'm a huge fan. I've read all the books many times over and I have all the British editions because the cover art is lively and crowded and the American versions are minimalist crap. So when Widge said he had two Discworld animated stories, Wyrd Sisters and , for review, I think my exact response was, "GIMME GIMME GIMME!"
The plot of Wyrd Sisters is largely an homage to William Shakespeare's tragedy Macbeth. The book's first line of dialogue is the first line of the play, in similar circumstances ("When shall we three meet again?", spoken by one of three witches on a stormy night). Further references include Duke Felmet's desperate attempts to wash the blood from his hand.