Clever presentation of the basics of how conspiracy theories begin, are substantiated, and then can (often) be refuted by zooming out and regaining a little perspective.
There’s a subplot on how the number of letters in a document proves the Templars possession of the Holy Grail. A character then shows how a random shopping list follows a similar pattern.
It's also fun to read Umberto Eco's take on Dan Brown and his Da Vinci's Code. Eco was joking that he feels he should get some credit for the whole thing, as Dan Brown is so clearly just a character from Foucault's Pendulum.
There's a clear lineage of bonkers conspiracy theories getting more popularly approachable from Illuminatus! to Foucault's Pendulum to Dan Brown.
Every 15 years or so, we get a new, more popularly approachable entry in the "Templars/Illuminati are real" mystery thriller genre. We're about due another one, but I'll bet I missed it because it's in the Young Adult Fiction section.
I think this is exactly why it is so maddeningly hard to engage critically with those who espouse these theories – they keep an 'authoritative' death-grip on the frame.