Thornton sets out to correct an imbalance in Atlantic history, where the crossing of Europeans has drawn far more attention than the larger African migration that accompanied it. Working outward from Braudel's idea of a region defined by its sea, he treats Africa not as a passive supplier but as a set of societies whose rulers and merchants set the terms of contact.

The first half examines commerce, warfare and politics on the African coast. European goods, Thornton concludes, were largely luxuries that neither displaced local industry nor made Atlantic trade necessary to African prosperity, and Europeans lacked the military strength to compel any exchange, including the traffic in captives. Slavery was already embedded in African legal and political life, which allowed elites to push its costs onto poorer neighbours.

The second half turns to the Americas, arguing that transplanted Africans had room enough to shape religion, revolt, language and maroon settlement, often extending changes that had begun at home.