It seems that the main "thesis" is based on a "fact" defended by the Priory of Sion... that got revealed several years later as being a complete fraud built up by the guys who funded the Priory.
But Dan Brown seems to be convinced (or at least claim so) that it was a real thing.
(and I still haven't read the book. I'm paralized with not caring very much)
WEEEEE, they filmed some scenes in Paris! I know these places now!
ETA : and the documentary is hosted by Baldrick. I mean Tony Robinson. I bet he's the best person to uncover a cunning plan.