Volume 1, Book 8, Chapter 1 opens with a flashback to the time just before Tristram was born, which seems to happen before the events of Book 7:
It was a king that hight Meliodas, and he was lord and king of the country of Liones, and this Meliodas was a likely knight as any was that time living. And by fortune he wedded King Mark’s sister of Cornwall, and she was called Elizabeth, that was called both good and fair.
And at that time King Arthur reigned, and he was whole king of England, Wales, and Scotland, and of many other realms...
The use of a flashback helps Malory thread together the disparate pieces of lore he is working with into one narrative. Whereas stories about King Arthur and the various knights of the Round Table had previously been told as separate, standalone tales set in the same world and borrowing some of the same characters, Malory puts them all together in a narrative that is (for the most part) cohesive. The occasional contradiction slips in, such as the two origin stories for Excalibur. But Malory does his best to explain how, for instance, his version of Tristram fits into his version of Launcelot's story.
The challenge is that Tristram's story takes place largely in France, a bit removed from the immediate vicinity of Arthur's court. This flashback at the opening to Book 8 zooms out from King Arthur's court to offer the reader a map of his whole territory and the surrounding area, including the place where Tristram is born. The flashback then goes on to describe how Tristram's life is shaped from birth by the politics of this map. By giving the reader this historical explanation for how Tristram relates to what has been going on with Sir Gareth, Gawaine, Arthur, and others, Malory irons over the seam between one part of the book and the next. In addition, he gives the entire book a sense of political and historical depth.
