Da Vinci Code copyright infringement?

I confess that when I started to read this I was surprised and suspicious of the litgants' motives. Now, you will recall that I think the DaVinci code is unfair and misleading, but I think that there is something odd about the authors of Holy Blood, Holy Grail suing Dan Brown for copyright infringement:

The case is also likely to clarify existing copyright laws over the extent to which an author can use other people's research. The non-fiction work deals with theories that Jesus and Mary Magdalene married, had a child, and that the blood line continues to this day - with the Catholic Church trying suppress the discovery. It is similar to the theme explored in The Da Vinci Code,

The oddness about this is that the authors Michael Baigent and Richard Leigh are claiming that the American novelist Dan Brown appropriated themes and ideas they explored in their 1982 book. Now I understood that they were offering their ideas as facts. Can facts be copyrighted? Or theories? Or even beliefs? I would have thought that the only way they could be successful would be to admit that their work was fiction... I must be missing something. This may be an interesting story to watch.
Guardian Unlimited Books | News | Da Vinci Code author begins copyright battle
Filed in: DaVinci_Code, copyright, book