Subtle allusions to the silver branch recur in other novels in the Eagle of the Ninth series, and it presumably refers to the otherworldly musical instrument mentioned in the medieval Irish narrative The Voyage of Bran.

The Silver Branch is second in Sutcliff's Roman Britain Series, following The Eagle of the Ninth (1954) and preceding Frontier Wolf (1980), The Lantern Bearers (1959), and Sword at Sunset (1963).

Justin (Tiberius Lucius Justinianus), a shy young army surgeon who had just completed his apprenticeship in Beersheba, is posted to Britain for the first time, although his family has been connected with the island for years.

Upon overhearing a plot against Carausius by his finance minister Allectus, they try to warn the emperor but, due to their perceived disloyalty, are instead reposted to Magnis on the Northern Wall in seeming disgrace, but confusingly combined with a moderate promotion.

When they are hiding at their great-aunt Honoria's place in Calleva, they discover the lost eagle standard that was buried by their ancestor, Aquila, around which their small group rallies.