When a treacherous act of murder throws the western provinces into turmoil, Aurelius Castus is ordered to take command of the military forces on the Rhine.

But he soon discovers that the frontier is a place where the boundaries between civilisation and barbarism, freedom and slavery, honour and treason have little meaning.

At the very heart of the conflict are two vulnerable boys. One is Emperor Constantine's young heir, Crispus. The other is Castus's own beloved son, Sabinus. Only Castus stands between them and men who would kill them. With all that he loves in danger, Castus and a handful of loyal men must fight to defend the Roman Empire.

But in the heat of battle, can he distinguish friend from enemy?