The Rise and Fall of the Celtic rulers of Britain, in Wace as in his Latin sources, takes place in four phases:

These parts are linked together by the theme of Brittany. Conquered by the Emperor Maximien in Part 1 and granted by him to Conan who founds a powerful dynasty there (at considerable expense for Britain), it becomes, in the words of Elizabeth Bryan, a ‘royal nursery’ for Britain. King Constantine, Arthur's grandfather, is the brother of the king of Brittany, and when Vortigern usurps the power, the young princes Aurelius and Uther seek refuge there. Arthur maintains strong bonds with his Breton kinsmen, in particular his cousin Hoel who is at his side throughout his campaigns. And in the final phase, it is to Brittany that King Cadwallader flees from the plague devastating his land, and in Brittany again that he has the divine vision telling him to renounce England.

The initial phase in the history of the Celtic kings of Britain, in Wace as in his sources, revolves around the relationship between Britain and Rome. This relationship is expressed on three levels: the common origins of Britons and Romans; the military campaigns opposing Britain and Rome; and the bond of Christianity between the two nations.

The myth of origins

The myth of the Trojan origins of the Britons is of crucial importance for an understanding of the development of the plot. The French poem presents §6 (the first chapter after the prolegomena) of the Latin as follows: Eneas becomes king (9–66); length of Eneas's reign, his achievements, his posthumous son, Aschanius's regency, his pedigree, achievements and duration of his rule (67–106); the conditions of Brutus's birth (107–48); Brutus's exile to Greece and his meeting the enslaved Trojans (149–60).