It’s hard to understate the importance of ‘The History of the Kings of Britain’ (completed by 1136) to the Matter of Britain. Before Geoffrey there are scattered stories, anecdotes and incidents. After Geoffrey, the swirling fragments become stories set in sequence, given a spurious feeling of historical truth.
The scale of the work is as easily overlooked as was the scale of the writer’s ambition and achievement.
He may not have created the foundation myth of British history, but he made it work as a coherent story. Beginning with the legendary Brutus, he notes every King who succeeded down to the historical Kings like Edwin and Oswald. It has very little historical value but the originality and inventiveness, the artistic shaping of the narrative, and the effort that must have gone into it are impressive.
His simple chronicle framework was very flexible. Some kings could be nothing more than names in a list:
Rivallo’s son Gurgustius succeeded him, Sisillius came after Gurgustius, then Jago the nephew of Gurgustius, then Kimarcus, the son of Sisillius and after him Gorboduc. (p. 87-88)
The frame also allowed for expansion, most obviously with Arthur, but also with Lier, Ebruac, Bellinus and Brennus and Vortigern.
What the chronicle framework imposed on individual stories nested between others, was the necessity for a narrative arc. Each had to have its own beginning and end, but then had to fit into the sequence between preceding and succeeding reigns.
The sequencing of events demands some form of causality: because this happened, that followed. Or the sequence will imply causality. This has happened, therefore what came before it was in some way responsible.
Even the most superlative imagination could not avoid repetition. The genre itself is built on it:
A king comes to the throne, rules, dies. Repeat.
It’s inevitable that patterns begin to appear and create a sense that the story is generating its own criteria against which actions can be measured.
Laȝamon, reading Wace, sees that love stories are stories of aberrant behaviour. There’s nothing heroic or tragic about his lovers.
For Laȝamon, one paradigm is begun when a King, seeing a woman, puts his desire for her before his duties and obligations. When we encounter Vortigern, lusting after Rowena, we’ve already heard the story of Locrin and Aestrild and know how badly that ended. What has happened casts its shadow forward over this new event. But it also works retrospectively. The story of Locrin will qualify the story of Vortigern, and the story of Hengist’s daughter will qualify the earlier story of Aestrild. Both will qualify the story of Uther and Ygaearne, which is complicated enough already.
The pattern should also set the alarms ringing when Arthur does something all for the love of Gweneviere.
And it's worth noting that only in the last of the four stories is the woman in any way responsible for the disaster that follows.