Maskerade (Discworld Novel 18)

This isn't real life - it's worse. This is the Opera House, Ankh-Morpork... a huge, rambling building where innocent young sopranos are being targeted by a strangely familiar evil mastermind in a mask and evening dress and with a penchant for lurking in shadows and occasional murder.

But Granny Weatherwax, Discworld's most formidable witch, is in the audience. And she doesn't hold with that sort of thing. There's going to be trouble (but nevertheless a good evenin's entertainment with murders you can really hum to) and the show Must go on.