During the Roman War, Lancelot and his kin have distinguished themselves as promising young knights, and the colophon promises “here folowyth afftir many noble talys of sir Launcelot de Lake” (247.6–7). This promise is fulfilled in “The Tale of Sir Launcelot,” which is mainly based on the Vulgate Lancelot but represents a departure from the way Malory adapted the sources of the previous two works. “The Tale of Launcelot” appears in approximately the same relative place as the Lancelot does in the Vulgate Cycle: after the Merlin and immediately after the Roman War. Yet in other respects, Malory's tale and the Vulgate version are very different. Instead of following most of his major source as in his first two tales, Malory unravelled a single narrative strand from a small part of the Lancelot, and he took an episode from another Old French prose romance, the Perlesvaus. “The Tale of Launcelot” is therefore not a redaction of a large part of a single source but a composite.

The Prose Lancelot is the oldest part of the Vulgate Cycle and, like the Prose Merlin, is thought also to have had a pre-cyclic life. This monumental romance begins during Lancelot's childhood with the loss of the kingdoms of Lancelot's father, King Ban, and his uncle, King Bors, to the villainous King Claudas. Lancelot, his brother, and their cousins are abducted by the Lady of the Lake, who raises them and has them trained to become knights.