This summary of the Lancelot focuses on Part 3 and is succinct with regard to Part 1 and brief with regard to Part 2. It will be followed by a résumé of the Queste and Mort Artu, and is preceded by information on the contents of the texts preceding the Lancelot in the Vulgate Cycle.

The Estoire del Saint Graal (S I), a text coming directly from Christ according to its prologue, describes how the Grail (the dish of the Last Supper, also used to collect the blood of the crucified Christ) came into the possession of Joseph of Arimathea and how it helped him during a long imprisonment and a prolonged stay with his followers in the desert. Joseph creates the Grail table. His son Josephé takes the Grail to Britain and on his way (in Sarras, for instance) converts many a heathen king to Christianity. Joseph's nephew Alain becomes the first keeper of the Grail. He is the Fisher King and builds the Grail castle Corbenic in the ‘Terre Foraine’, where his successors will await the Grail quest.

In the Estoire de Merlin (S II, 1–88), a devil makes a pious girl pregnant. She gives birth to Merlin, whose devil father has given him knowledge of the past, whereas God has bestowed on him knowledge of the future. He will use his gifts to predict the downfall of the usurper Vortigern and to help Uther to power and into the bed of Ygraine, where Uther will father Arthur. Merlin's knowledge is the basis of his own story, which he dictates to his faithful scribe Blaise. For Uther, Merlin brings the Giant's Ring (Stonehenge) from Ireland as a fitting monument for Uther's brother Pandragon. He also creates the Round Table, as the third table in the sequence of the table of the Last Supper and Joseph's Grail table. Merlin takes care of Arthur's upbringing. The episode of the sword in the stone makes clear that Arthur is the rightful inheritor of Uther's crown.

The barons of his realm do not accept Arthur's rule; the Suite-Vulgate du Merlin (S II, 88–464) describes Arthur's struggle for power during long years of internal strife. Arthur marries Guinevere and his father-in-law gives him the Round Table as a wedding gift.