Maskerade

It's true, there's a ghost in the Opera House of Ankh Morpork. Sardonic, flamboyant and, well, ghostly, it wears a bone-white Mask and terrorizes the entire company, including the immortal Enrico Basilica, who eats continuously even when he's singing. Mostly spaghetti with tomato sauce.
What better way to flush out a ghost than with a witch? Or even two! And Granny Weatherwax and Nanny Ogg happen to be in Discworld's capital city trying to recruit a third (since three witches make a coven, and two make only an argument).
Enter the Opera's newest diva, the alarmingly fetching Perdita X. Nitt, who has such an astonishing range that she can sing harmony with herself, and is so agreeably large that she hangs out with the elephants in the cellar.
They say that inside every fat woman there's a thin woman struggling to get out (or at least dying for chocolate). In Perdita's case, the thin woman is more ambitious, since she would also dearly love to be a witch.
Beginning to get the picture? One would hope so. For this isn't cheese, it's opera, which runs on a Catastrophe Curve. And to further complicate matters, there is a backstage cat named Greebo who occasionally becomes a person just because it's so easy. Not to mention Granny Weatherwax's old friend, Death, whose scythe arm is sore from so much use.
And who has been known to don a Mask ...