Mercury: a planet of two faces – one side cloaked in eternal darkness, the other blistered by eternal sunshine. And balanced between the two is the World River, encircling the globe.

Professor Nathanial Stone and his ward, Annabelle Somerset visit Mercury for what they imagine will be a pleasant trip to visit Annabelle's uncle.

But tragic death precedes them – a death that provokes a chain of mysterious and terrifying events that challenge Nathanial and Annabelle's long-held beliefs. Before long, they – with the assistance of French geologist, Arnaud Fontaine – discover something ancient and powerful that has designs on humanity.