Holy Blood, Holy Grail is the title of a book, originally published in 1982 by authors Michael Baigent, Richard Leigh, & Henry Lincoln. The hypothesis of the book is essentially the underlying story of the popular book The Da Vinci Code, by Dan Brown. According to Brown, Jesus was married to Mary Magdalene and had at least one child with her. Mary Magdalene, their children, and perhaps even Jesus Himself moved to France, and eventually intermarried with the Frankish Merovingian dynasty. This truth was erased and covered up by the Roman Catholic Church, who wanted to preserve the Church’s authority through apostolic succession from the Apostle Peter, instead of through Jesus’ actual bloodline. The "Holy Blood" is the line of Jesus’ descendants. The "Holy Grail" is Mary Magdalene, who carried Jesus’ blood inside of her.
All of the central themes of The Da Vinci Code can be found in Holy Blood, Holy Grail. The Knights Templar, the Priory of Sion, Opus Dei, the cover-up at the Council of Nicea, etc. are all presented in Holy Blood, Holy Grail and then fictionalized in The Da Vinci Code. There are so many similarities, in fact, that the authors of Holy Blood, Holy Grail sued Dan Brown, the author of The Da Vinci Code for plagiarism.
Is there any truth to the theories brought up in Holy Blood, Holy Grail? Like most conspiracy theories, Holy Blood, Holy Grail is long on conspiracy and utterly lacking in evidence. Even non-Christian scholars, with no reason to defend the historical Christian viewpoint, have labeled Holy Blood, Holy Grail as a "pseudohistory," and a baseless concoction of the authors. Even one of the authors of Holy Blood, Holy Grail admitted that their goal was to present a "plausible hypothesis," but that none of them actually believed it to be true. May we all follow their example, and recognize the completely fabricated nature of Holy Blood, Holy Grail and The Da Vinci Code.