Retired archaeologist Judith Potts lives alone in a faded mansion in the peaceful town of Marlow, filling her time by setting crosswords for the local paper. During one of her regular wild swims in the Thames, Judith hears a gunshot coming from a neighbour's garden and believes a brutal murder has taken place.

In a world cleaved in two by a massive barrier of perpetual darkness, a young soldier uncovers a power that might finally unite her country. But as she struggles to hone her power, dangerous forces plot against her. Thugs, thieves, assassins and saints are at war now, and it will take more than magic to survive.