Throughout the story there are references to the ongoing debates in operatic circles between on one hand the traditional large women who can sing but don't look anything like the character they are playing - the 50 year old "fat lady" playing the part of the consumptive young heroine - and on the other hand the young thin woman who looks the part but doesn't have the lungs to sing the role.
Agnes Nitt, a girl from Lancre fits the role of the large buxom traditional looking opera star while Christine who is patterned after "Christine Daae" from the Phantom fits the more modern opera soprano style. The story begins on the heath with the two Wittches Granny Weatherwax and Nanny Ogg as in Macbeth discussing when they will meet again and realizing that a coven needs three witches not two.
Since they are one short after the departure of Magrat Garlick who left to become Queen of Lancre, they journey to Ankh-Morpork to bring Agnes Nitt back to join their coven as well as to obtain the money owed to Nanny Ogg by the publisher of her book, The Joye of Snacks. Agnes, who has a beautiful singing voice has left Lancre to seek a career in music, singing in the taverns before joining the Ankh-Morpork Opera company, which is being haunted by a ghost, to sing in the chorus.
Here she meets Christine , a thin talentless girl who the Ghost of the Opera House seeks to make the new diva. Agnes, who has the talent to be the new diva is seen as too fat. Since Christine can't sing, Agnes Nitt is used as her backup voice, giving Christine the spotlight. With the money from her book rightfully coerced out of the publisher, Granny Weatherwax pretends to be a rich patron to catch the Opera Ghost.
Near the end of the novel, Andre, a member of the Cable Street Particulars, along with the Witches determine that the finances of the Opera House are deliberately kept in a total mess to disguise the money-laundering going on, and the murders and 'accidents' were caused to distract the manager from investigating the matter properly. It is revealed that there were two people pretending to be the ghost, the original Ghost was the janitor's son who wished to watch the Opera, while the second Ghost did it to steal the money. Christine - the thin lead soprano in the company who cannot sing is a take off on Christine Daae, the lead in the Phantom of the Opera.
In the original Phantom, Christine is to sing the lead in Faust, which is the title of one of Pratchett's earlier works.