The author provides an in-depth reassessment of the life of Magnus Maximus, Roman emperor ruling in the west from 383 to 388 using the Classical sources and archaeology, and of the quite different impression of his life as reflected in the post-Roman to Medieval British insular sources.

While most historians tend to dismiss Maximus as an ephemeral usurper, his time in the sun shows every sign of having been a success. He cast a long shadow in Britain, where he was originally proclaimed. Yet early non-Roman sources, notably Gildas, condemn him for leaving the island bereft of defenses due to his usurpation. In contrast subsequent ones cast him as the progenitor of several British dynasties on the frontiers, while the Medieval Mabinogion story, The Dream of Maxen Wledig, presents him as an all-conquering figure of Romance who allied himself with a powerful British dynasty and facilitated the settlement of Brittany by the British.

Following an introductory account of Roman Britain, its troubles and imperial adventures from Clodius Albinus in 193 to the end of the so-called "barbarian conspiracy" in 368, Maxwell Craven examines all the sources to show how important the ardent Christian Maximus was to the settlement of the British frontiers. It was his work that kept the British tribes from being overwhelmed by Germanic invaders during the following centuries.