The book Holy Blood, Holy Grail was published in 1981 or 1982. The authors are Michael Baigent, Henry Lincoln, and Richard Leigh.
These 3 authors discovered a story of a supposed treasure in Rennes-le-Chateau, France, and the priest who is claimed to have found the treasure in the 1890s and become wildly rich beyond all imagination. In their research into the mysteries of the priest and the church and the treasure, the 3 authors found links to a man name Pierre Plantard and an organization called the Priory of Sion. There were even documents located in the French national library regarding the Priory.
It was Pierre Plantard who told Baigent, Leigh, and Lincoln that the treasure found by the priest was documents that proved Mary Magdalen and Jesus were married and that after the crucifixion she settled in or near Rennes-le-Chateau to give birth to their child. The Holy Grail, according to Plantard, was an error in understanding the words Sang Raal - Holy Blood. When it is misspelled as San Graal, the meaning changes to Holy Grail. And according to Plantard, the Holy Blood was the bloodline of Jesus and the Grail itself was the womb of Mary Magdalen.
Ready for the rest of it?
Pierre Plantard had forged all the documents that were found in the French national library and placed them there in the 1950s. He tried to interest other men in becoming part of his "Priory of Sion" which wasn't to find or protect any of the descendants of Jesus - their goal was to restore a monarchy to France. The group never really got off the ground.
Before he died, Plantard had to testify in court that his documents were false, that there was no Priory of Sion except in his mind, and that there was no connection between his Priory and the original Priory, which disbanded 800 or more years ago. He admitted in court that his story about Jesus and Mary Magdalen was all a fraud.
So is The da Vinci Code true? No, not even remotely. And even if it were, there isn't any way that birth and marriage and death records have been maintained in completeness, with full accuracy, for 2000 years or more. There would never be any way to validate anyone's claims to be a descendant of Jesus through the Merovingian kings of France.
There are thousands of web sites that give you the entire story of Plantard and Rennes-le-Chateau. Although many are supportive of the fictions, there are a LARGE number of sites that offer page after page after page of documentation debunking the entire story.
Just Google "Pierre Plantard" or "Rennes-le-Chateau" or "Merovingian" and you can read it all for yourself.