What is this show about? (Might contain spoilers)

The show is suppose to be based on a book called Labyrinth by Kate Mosse. I have not read the book, but I have watched the show.

In the most simplest of terms, the plot revolves around the 'holy grail'. It has conspiracy, cults, and such. I'd hate to compare it to The Da Vinci Code but they are quite similar, except the show has a bit more story and less action. Now here is a soft spoiler: the grail in The Da Vince Code is NOT the same as in this show.