The witches Granny Weatherwax and Nanny Ogg visit the Ankh-Morpork Opera House to find Agnes Nitt and get caught up in a story similar to The Phantom of the Opera.  A significant character in the book is Walter Plinge, the Opera House's clumsy and not-too-bright odd-job man. He is, quite possibly, really the suave "Opera Ghost", but is described in terms reminiscent of Frank Spencer, the lead character of the BBC sitcom Some Mothers Do 'Ave 'Em. The joke is that this character was played by Michael Crawford, who went on to play the Phantom in the Andrew Lloyd-Webber musical.  The title is a pun on Masquerade, a key song in the musical.

Wyrd Sisters was about theatre, this is about opera and it's a mix of comic absurdity and zinging hot characterisation (not to mention Nanny Ogg's erotic cookbook). Basically Nanny and Granny are missing a witch, now that Magrat's off queening there's a space in the coven and tempers are fraying, more than usual. However Agnes (from Lords and Ladies and the intended taget/victim) has other ideas of her own and she decides to try and have a life of her own. Agnes is caught up in a world where art imitates life and nothign is quite what it appears to be. Luckily Granny and Nanny arrive to mess things up and also sort them out again. I think Agnes is an interesting new addition to the mix, and the scene with Death and the dying swan really is far more interesting than it seems.