The book is a follow up to Witches Abroad. The witches Granny Weatherwax, Nanny Ogg, and Magrat Garlick return home to the little mountain kingdom of Lancre, to discover that amateur practitioners in magic have weakened the fabric of reality. Elves manage to break through, and they must be defeated.  The plot turns on Magrat's wedding to King Verence II, and her misgivings about it. The climax takes place on the day before the wedding. The plot includes several elements of A Midsummer Night's Dream by William Shakespeare, and quotes lines from various folk songs about elves.  The sub-plot includes the Unseen University wizards, mainly Archchancellor Mustrum Ridcully, Ponder Stibbons, the Bursar and the Librarian.  According to Pratchett himself, this is one of the few Discworld novels that cannot stand on its own; i.e. one needs to have read the previous two books in the Witches sub-series, viz. Wyrd Sisters and Witches Abroad, to have a proper understanding of the characters and their relation to each other.

Elves are not the nice, kind, guardians of truth and justice seen in many popular films, they're basically malevolant monsters who are cruel beyond belief and believe that torture is a perfectly normal way to pass the time. The witches of Lancre must stand up to them, except Magrat's distracted by a wedding she's not sure she's ready for yet and even then it may not be the one she wants. Granny is worried that her time is up and starts preparing for her end and Nanny has a bath, much to the shock and distress of her neighbours! In the end though all three witches put their differences aside and work together to save the day. There's room too for Magrat's eventual replacement...