I realize that The Da Vinci Code was just a book/movie, but it was largely based on fact. Anyone have detals?

I would say that while the Da Vinci Code was highly entertaining, it was certainly not based on fact. It was based on a theory that is somewhat popular with conspiracy theorists but not highly regarded in academic circles. The book Holy Blood Holy Grail by Michael Baigent, Richard Leigh, and Henry Lincoln was the first time this theory was published. The History Channel has aired a variety of doentaries exploring this theory, including one where they tried to use modern DNA technology to prove that the Marivengian (I'm sorry if I spelled this wrong) royal line in europe may have been from Christ. They were not able to prove it.