The Silver Branch is set in the third century during the Roman invasion of Britain. The main characters are two cousins, Justin who is an army surgeon and Flavius who is a soldier. The cousins serve under Emperor Carausius who rules over Gaul and Britain. Flavius and Justin meet for the first time when Justin is assigned to serve at the Rutupiae fort on the coast of Britain. The young men uncover a secret plot to kill the emperor headed by the finance minister, Allectus. Justin and Flavius attempt to warn Carausius but he assigns them to duty on the Northern wall which they think is an insult but they report anyway. While there Justin and Flavius meet a local man named Evicatos of the Spear who tells them that the plot to kill the emperor is legitimate because has heard that news from his own people. Thinking that they must do whatever they can to save the emperor, Justin and Flavius decide to sail to Gaul to seek the aid of the ruler Constantius. Before they are able to sail, they meet a man named Paulinus who works underground to save other men who are targets of the evil Allectus. Justin and Flavius decide that their efforts would be better spent helping Paulinus in his work so they remain in Britain. Paulinus is ultimately murdered by Paulinus' men so Justin and Flavius travel to their Aunt Honoria's home to hide for awhile. It is there that they find a broken Roman eagle standard which had been lost in a campaign over one hundred years before. Justin and Flavius adopt the eagle as a rallying icon to rally recruits to build a legion of their own to go against Allectus. Constantius does finally arrive in Britain and Justin and Flavius' region join him in defeating Allectus' forces. Constantius forgives Justin and Flavius for leaving their posts on the North Wall and discharges from military service. However, Justin and Flavius feel that they owe their loyalty now to Constantius so they join his forces as they advance into Britain.