This is in part a 13th story of the evangelism of Britain as written by an author who knew almost nothing about Britain or about the few traditions on this head which the Church thought it knew.
The author's sources were in part the 'Joseph of Arimathea' written by the late 12th-century or early 13th century author named Robert de Boron and in part fragments of a supposed early history of the grail embedded in the 13th century romance 'The Quest of the Holy Grail'. A prose version Robert de Boron's work appears along with its sequel the 'Story of Merlin' appears as a prequel to the 'Prose Lancelot' in some medieval texts but there are problems in marking its continuity fit with the 'Prose Lancelot' and the 'Quest of the Holy Grail'.
For one thing, Robert de Boron seems to have imagined that Arthur began to reign about a generation after Christ while the 'Prose Lancelot' and the 'Quest of the Holy Grail' place Arthur in the 5th century following the birth of Christ based on supposedly historical chronicles. It would be impossible (although Robert de Boron seems to indicate it) that Perceval was the son of Joseph of Arimathea's nephew Alain/Alan the Stout unless Alain's life was amazingly prolonged. Instead, the 'Quest of the Holy Grail' only mentions a certain Alain/Alan the Stout who is a distant ancestor of Lancelot.
Where Robert de Boron makes out that Joseph of Arimathea's brother-in-law Bron is the Rich Fisher, that is apparently the Fisher King of the grail stories, the 'Quest of the Holy Grail' makes no mention of either Bron for himself or as the father of Alain the Stout.
Accordingly the author of the 'History of the Holy Grail' creates a saint-life pastiche mostly based on the 'Quest of the Holy Grail' but includes modified versions of Robert de Boron's history. It is not mentioned, perhaps left out by accident, that Bron is the husband of Joseph of Arimathea's sister and that Alain the Stout is thus Joseph's nephew. Rather the story of how Bron miraculously caught a fish is applied to Alain the Stout who becomes instead of Bron the first keeper of the grail after the death of Josephus son of Joseph of Arimathea. Josephus was not even mentioned by Robert de Boron. Robert de Boron makes out that Alain is celibate which makes it difficult to see how he could be the father of Perceval. The author of the 'History' keeps Alain celibate and when he dies, the grail passes to his brother King Joshua, unknown to Robert de Boron, who becomes the father of a line of kings known as the Fisher Kings who guard the grail. The author distinguishes Alain the Stout who was a grail keeper from the King Alain the Stout who is a descendant of Nascien and an ancestor of Lancelot.
The author had other characters to account for. Robert de Boron introduced a follower of Joseph of Arimthea named Petrus who is supposed to, in the future, take a certain divine letter to the grandson of Bron in Britain. Instead, the author changes Petus to Pierre (Peter) and tells a romance about him which leads to him marrying the Daughter of the Queen of Orkney and so through a series of names becoming the ancestor of King Lot, the father of Gawain. Galahad, the son of Joseph of Arimathea is mentioned in the 'Prose Lancelot' and seems to be intended to be an ancestor of Lancelot, although this is never clearly said in the 'Prose Lancelot'. The author instead makes this Galahad to be the ancestor of Kings of Wales from whom descends King Urien the father of the Arthurian knight Yvain, Yvain being arguably Arthur's best knight after Lancelot and his kin and after Gawain son of Lot and Gawain's brother Gaheriet.
The author's story is the tale of how Joseph of Arimathea obtained the wonder-working dish from which Jesus had eaten at the last supper, for French 'graal' means 'dish' or 'platter', not 'chalice'. The author then relates how he converted King Evalach, the king of a Middle-eastern country near Egypt, to Chistianity, and how Joseph's son Josephus was then appointed by God himself into the 'first bishop of Christendom', suggesting strongly a line of bishops outside of the later Roman church and more prestigious. Then Nascien (originally named Seraphe), the brother-in-law of Evalach is introduced as the greatest champion of his age and the one who is to become ancestor of Lancelot.
The author's main purpose is make a tale which will agree with that of the 'Quest of the Holy Grail', will agree with some hints in the 'Prose Lancelot' and will agree with much of Robert de Boron's 'Joseph of Arimathea'.
There are hints of pagan Celtic mythology in this story. Bron may be in origin identical to the god Brân who has become a humanized pre-Christian king in the second branch of the Welsh 'Mabinogion'. Evalach may be the Welsh ancestor figure Avallach, in Welsh tradition supposedly the son of a certain Beli who had married Anna, here the cousin of the Virgin Mary. Beli also seems to have been originally a god and 'Avallach' is used in Welsh translations of French and Latin Arthurian texts for 'Avalon', the mysterious island to which the wounded Arthur was taken after his last battle. Avallach appears in person as the ancestor of the British kings of the North of Britain and the father of the goddess Modron who was originally identical or later confused with Morgain the Fay.
The Evalach of this tale attempts to look too closely into the grail and so earns the wrath of God. He is blinded and wounded and will lie in torment in a monastery until the promised knight who will achieve the grail will come to him and he will die in his arms. This the author took from 'The Quest of the Holy Grail'. But he adds that Evalach was in origin a commoner, originally a simple cobbler of Meaux, one of the many extra bits of information he adds of unknown source.
The world of which the author writes is one in which garish and extravagant miracles abound and so there is no real difficulty in proving by these miracles that Christianity is the true religion. Josephus takes off his tunic, miraculously expands it in size, and he and his company ride it over the waves from the Middle-east to Britain where they settle in Northumberland, in an otherwise unknown city of Galafort, supposedly not far from Oxford in the fantastic geography of this saintly romance. There is no mention whatsoever of Glastonbury. (Similarly, surviving traditions of Glastonbury do not mention the grail, but claim that Joseph of Arimathea who founded a church at Glastonbury came with two cruets containing the blood of Christ.)
The author only once messes up badly, he has Pellehan, the grail king of the 'Quest of the Holy Grail', wounded between the thighs in a unexplained 'battle of Rome' instead of having Pellehan wounded by a miraculous lance after he has attempted to draw the sword in the ship of Solomon as told in the 'Quest'. This is certainly just an error but is a puzzling one.
The author claims to have lived 717 years after the Passion of Christ (that is in the year 750 assuming the Passion is assigned to the year 33 as is normal in medieval texts) and to have lived in a wild place in Britain He tells how Jesus appeared to him on Good Friday eve and gave him a book which Jesus had written himself, which told of the author's kin. This book was supposedly written after the resurrection and tells of Joseph of Arimathea and the grail and of the lineages that spring from Joseph and from Nascien. Jesus tells the author to make a copy. The 'History of the Holy Grail' that we possess is perhaps supposed to be a translation of that copy from Latin to French. Believe that if you wish.
In 1547 the Roman Catholic Church condemned Grail lore as a heresy.