The Elves have invaded Roundworld, and the wizards of Unseen University decide that they have to do something about it. Their first attempt turns London into a dozy Neanderthal village where nothing much has happened for the last million years. But with the aid of Isaac Newton, William Shakespeare, and a hitherto unknown Greek philosopher, anything can happen. And does.
        As usual, the Discworld action is punctuated by Very Big  Footnotes (thinly disguised as even-numbered chapters) about the underlying science (and art and religion and philosophy).