The book to me has three main thrusts to it. The first is to introduce us to as many of the Rougons and Macquarts as possible, giving a background to the series and some ideas as to how they all interlink.
The second is a love story between Miette and Silvere told against the backdrop of the Macquart family and the insurrection of 1851.
The third is the story of Pierre Rougon, eldest son of the series matriarch, and how his rise to power in Plassans was far from honourable.