In AD180 Quintus Suetonius and his stepson Manius are frumentarii, the Roman Empire's secret police.
Dispatched to Britannia on a mission whose purpose is from them they they join forces with the formidable female spy Tita Amatia and find themselves embroiled in plot, counter-plot and assassination, unsure of either their allies or their enemies.
They journey from bustling, cosmopolitan, civilised Londinium to the wild North - a hostile snowbound land.
Besieged by an alliance of rebellious lo.