Justinian's Torch follows Cillian in his third summer in Caledonia at the northern end of Britannia with the army of Belisarius. Two years earlier, Emperor Justinian sent Belisarius to conquer the Caledonii [Picts], something no Roman had ever done. But as the army approached Caledonia, the Erunai [Irish] of Dalriada—an Eruain colony on Caledonia's west coast—attacked both the Caledonii and Romans. In response, the Romans and Caledonii allied and have pushed the Dalriadae back into their highland stronghold of Dalriada.

The Romans and Caledonii are about to invade Dalriada and Cillian now commands a century of Eagle class airships and nine tent groups of sagittarius, light-infantry archers, in the 4th Airborne Legion that supports Belisarius's army. He faces not only the brutal, clever tactics of the Dalriadae but coming to terms with the discovery and implications of his own Eruain lineage. Cillian must also deal with the Caledonian Druidess, Coevla, and her prophecy that would condemn him to death if the emperor heard of it. And then there is the challenge of flying airships in the wet and windy highlands of Dalriada. While victory over the Dalriadae this summer seems certain, Cillian wonders what that victory will cost him and his century.