In October 1993 our narrator, a novelist, is invited to go to Stockholm and then to Russia to take part in what is enigmatically referred to as The Diderot Project. While in Stockholm he is joined by various other members of the Project -- an academic aptly named Verso whose nickname was The Encyclopaedia, and a lustful opera-singer. As we journey towards Russia we find out more about the main subject Diderot, for example how he was the son of a knife-maker in Langres who went to Paris and compiled a book that changed the world, The Encyclopaedia. In a series of dual narratives, one contemporary and one two hundred years earlier, Bradbury brilliantly recreates the climate of the eighteenth century and Diderot's journey to Russia to entertain and illuminate the mind of Catherine the Great, the most powerful monarch, whose influence could change the path of history.We learn how he could be seen as the godfather of both the modern novel and of the computer, and how there might be an extraordinary amount of missing material in St Petersburg. The Diderot Project becomes a quest to recapture Diderot's lost world, and in so doing illuminate ours.