This book provides more or less the complete story of the alliterative poem also called The Death of Arthur. I have a separate version of that, and it’s an interesting read in its own right, but here it’s just another episode to be Frankensteined onto the structure.
It’s a short book, and surprisingly little happens in it. I should perhaps mention that the entire thing is wildly ahistorical? I find it interesting that this was a fantasy people wanted in the centuries after the end of the Western Empire. If I remember my reading correctly, early on in the post-Roman decades, a few British warlords took a stab at continental conquest. They had little success, but the idea was popular enough to have been projected into the Arthurian context and to have survived there in a few different versions.
The war with Rome is logically slotted into the timeline. Arthur has consolidated his power locally, and the Round Table has settled down. Twelve envoys arrive from Lucius, Emperor of Rome, demanding tribute, to which the response is basically, “No U.” Every king unites behind Arthur, who treats the envoys well and sends them back to Lucius laden with gifts and a command that they recognize Arthur as the rightful Emperor.
Lucius musters his armies and 50 giants, to boot, and sets out for Flanders. Arthur assigns two governors in his absence (neither of them is Mordred, although in other versions of the story his being left in charge while Arthur is off on this adventure is what provides Mordred his chance to take the throne). Arthur names Sir Constantine as his heir, leaves Gwen fainting at being left by both her husband and her lover, and the whole army takes ship.
Arthur has a peculiar dream while at sea to presage the next chapter, in which he fights a truly monstrous giant of the old-fashioned, child-eating, maiden-ravishing sort. This is the last real adventure he gets in Malory, so at least it’s a good one. I think of this book as the end of “part 1,” in which the stories have a distinctly older feel to them than the rest. They emphasize Arthur as a warrior, and are full of strange magic and inexplicable events. The later stories are more narratively sophisticated, but they reflect a very different world than the one we started in.
There follow a few pages of battles, but in short order Arthur has faced Lucius himself and killed him. Some of the cities fight on anyway, and the army makes slow progress toward Rome. Gawain gets a side quest in which he shows better than usual. He meets a Saracen named Priamus who eventually converts and joins the Round Table.
Arthur is crowned emperor by the Pope and comes back home, and nothing more is ever said about the whole adventure–partly because in Book 6, Lancelot shows up and more or less takes over the story for a while.