The ever-rising tide of sales of ''The Da Vinci Code'' has lifted some pretty odd boats, and none odder than the dodgy yet magisterial ''Holy Blood, Holy Grail,'' by Michael Baigent, Richard Leigh and Henry Lincoln. A best seller in the 1980's, ''Grail'' is climbing the paperback charts again on the strength of its relationship to Dan Brown's thriller (which has, in turn, inspired a crop of new nonfiction books coming out this spring, from ''Breaking the Da Vinci Code'' to ''Secrets of the Code: The Unauthorized Guide to the Mysteries Behind The Da Vinci Code''). ''The Da Vinci Code'' is one long chase scene in which the main characters flee a sinister Parisian policeman and an albino monk assassin, but its rudimentary suspense alone couldn't have made it a hit. At regular intervals, the book brings its pell-mell plot to a screeching halt and emits a pellet of information concerning a centuries-old conspiracy that purports to have preserved a tremendous secret about the roots of Christianity itself. This ''nonfiction'' material gives ''The Da Vinci Code'' its frisson of authenticity, and it's lifted from ''Holy Blood, Holy Grail,'' one of the all-time great works of pop pseudohistory. But what seems increasingly clear (to cop a favorite phrase from the authors of ''Grail'') is that ''The Da Vinci Code,'' like ''Holy Blood, Holy Grail,'' is based on a notorious hoax.
The back story to both books, like most conspiracy theories, is devilishly hard to summarize. Both narratives begin with a mystery that leads sleuths to vaster and more sinister intrigues. In Brown's novel, it's the murder of a curator at the Louvre; in ''Grail,'' it's the unusual affluence of a priest in a village in the south of France. In the late 1960's, Henry Lincoln, a British TV writer, became interested in Rennes-le-Chateau, a town that had become the French equivalent of Roswell or Loch Ness as a result of popular books by Gerard de Sède. De Sède promulgated a story about parchments supposedly found in a hollowed-out pillar by the town priest in the 1890s, parchments containing coded messages that the priest somehow parlayed into oodles of cash. Lincoln worked on several ''Unsolved Mysteries''-style documentaries about Rennes-le-Château, then enlisted Baigent and Leigh for a more in-depth investigation.
What eventually emerges from the welter of names, dates, maps and genealogical tables crammed into ''Holy Blood, Holy Grail'' is a yarn about a secret and hugely influential society called the Priory of Sion, founded in Jerusalem in 1099. This cabal is said to have guarded documents and other proof that Mary Magdalene was the wife of Jesus (who may or may not have died on the Cross) and that she carried his child with her when she fled to what is now France after the Crucifixion, becoming, figuratively, the Holy Grail in whom Jesus' blood was preserved. Their progeny intermarried with the locals, eventually founding the Merovingian dynasty of Frankish monarchs. Although deposed in the eighth century, the Merovingian lineage has not been lost; the Priory has kept watch over its descendants, awaiting an auspicious moment when it will reveal the astonishing truth and return the rightful monarch to the throne of France, or perhaps even a restored Holy Roman Empire.
All the usual suspects and accouterments of paranoid history get caught up in this 1,000-year jaunt: the Cathar heretics, the Knights Templar, the Rosicrucians, the Vatican, the Freemasons, Nazis, the Dead Sea Scrolls, the Protocols of the Elders of Zion, the Order of the Golden Dawn -- everyone but the Abominable Snowman seems to be in on the game. ''Holy Blood, Holy Grail'' is a masterpiece of insinuation and supposition, employing all the techniques of pseudohistory to symphonic effect, justifying this sleight of hand as an innovative scholarly technique called ''synthesis,'' previously considered too ''speculative'' by those whose thinking has been unduly shaped by the ''so-called Enlightenment of the 18th century.'' Comparing themselves to the reporters who uncovered the Watergate scandal, the authors maintain that ''only by such synthesis can one discern the underlying continuity, the unified and coherent fabric, which lies at the core of any historical problem.'' To do so, one must realize that ''it is not sufficient to confine oneself exclusively to facts.''
Thus liberated, Lincoln et al. concoct an argument that is not so much factual as fact-ish. Dozens of credible details are heaped up in order to provide a legitimizing cushion for rank nonsense. Unremarkable legends (that Merovingian kings were thought to have a healing touch, for example) are characterized as suggestive clues or puzzles demanding solution. Highly contested interpretations (that, say, an early Grail romance depicts the sacred object as being guarded by Templars) are presented as established truth. Sources -- such as the New Testament -- are qualified as ''questionable'' and derivative when they contradict the conspiracy theory, then microscopically scrutinized for inconsistencies that might support it. The authors spin one gossamer strand of conjecture over another, forming a web dense enough to create the illusion of solidity. Though bogus, it's an impressive piece of work.
Finally, though, the legitimacy of the Priory of Sion history rests on a cache of clippings and pseudonymous documents that even the authors of ''Holy Blood, Holy Grail'' suggest were planted in the Bibliotheque Nationale by a man named Pierre Plantard. As early as the 1970's, one of Plantard's confederates had admitted to helping him fabricate the materials, including genealogical tables portraying Plantard as a descendant of the Merovingians (and, presumably, of Jesus Christ) and a list of the Priory's past ''grand masters.'' This patently silly catalog of intellectual celebrities stars Botticelli, Isaac Newton, Jean Cocteau and, of course, Leonardo da Vinci -- and it's the same list Dan Brown trumpets, along with the alleged nine-century pedigree of the Priory, in the front matter for ''The Da Vinci Code,'' under the heading of ''Fact.'' Plantard, it eventually came out, was an inveterate rascal with a criminal record for fraud and affiliations with wartime anti-Semitic and right-wing groups. The actual Priory of Sion was a tiny, harmless group of like-minded friends formed in 1956.
Plantard's hoax was debunked by a series of (as yet untranslated) French books and a 1996 BBC documentary, but curiously enough, this set of shocking revelations hasn't proved as popular as the fantasia of ''Holy Blood, Holy Grail,'' or, for that matter, as ''The Da Vinci Code.'' The only thing more powerful than a worldwide conspiracy, it seems, is our desire to believe in one.