In a nutshell:

The book is about a theory that the Holy Grail is actually referring to Mary Magdaline, who supposedly had a relationship with Jesus. They produced a daughter - and their bloodline is protected by a Secret Order, starting with the Knights Templar. Leo DaVinci and many other great artists were members of this order. And in the book, the Catholic Church is involved in killing those who know the truth about Jesus because it would destroy their power.

Very good book. You should read it.