Where there's a CODE there's a COD ...
In the not so distant past, a man had a Very Cunning Theory which he wrote down in a book, and it proved to be very popular indeed. In the very distant past, a man was born who had a wife, who had a child, who grew up and had children in turn, and so there was a bloodline stretching to today. And Leonardo da Vinci knew all about it and recorded it in a painting for everyone to see, despite lots of Bad People trying to cover up the whole thing. And the man with the Very Cunning Theory figured all this out from lots of clues and pictures that Leonardo da Vinci had cunningly concealed, and then published it for everyone to see.
But then it turned out that this Very Cunning Theory was Not So Cunning After All because it wasn't true. Not even remotely. Not even a tiny bit likely. Which is where this book comes in.
Because it turns out that although Leonardo da Vinci didn't know anything at all about a holy bloodline extending to the present day, he knew a very great deal indeed about what cod really are, and that sinister knowledge is only now coming to light ...
Fishclaimer
This book has not been authorized or endorsed by Dan Brown or his publishers, but it is much, much funnier.