In the first two volumes, the wizards of Unseen University accidentally created a Universe, which fortunately fit inside a manageable container. In this Universe, they found what they called "Roundworld", a fictional(?) version of our Earth; and proceeded to save it, or rather its humans, from a variety of ills (such as an invasion of Elves. Discworld Elves are nasty critters).
As before, the format of this book is chapters of a Discworld novella featuring the wizards, alternating with chapters of science illustrating and illustrated by the fictive chapters.
The fictional part is easily described. Malign forces seek to prevent humanity surviving (by leaving the Earth before it becomes an iceball). They are doing this by sabotaging a key point of the scientific advancement - in particular, they have caused a Reverend Charles Darwin to write _Theology of Species_, which fully supports Paley's watch. So they have to figure out how this is happening, and maybe even who is behind it, and put things right.
The nonfiction chapters are not so easily described. They mostly cover evolutionary science, but also bits of cosmology (a fairly thorough treatment of the multiple universes interpretation of quantum theory wanders in), history, sociology, etc. But they are very readable and entertaining (to the mind interested in such things).