Copyright laws are very straightforward. How they are being interpreted though is a bit more Byzantine. Dan Brown, as is being universally reported, is in court in England on a charge of plagarizing Holy Blood, Holy Grail by Leigh and Baigent. In scholarly works one can lift ideas with proper attribution (such as a footnote or a textual note) so why should it be different with fiction? Copyright, at least in the U.S., protects completed works from being reproduced in whole or in part. Thus, you can't take sections of completed works, characters, settings, plot devices, etc. and use them in another work. In the Dan Brown case, however, Leigh and Baigent are suing based on Brown's use of their speculative theory that Jesus and Mary Magdalene were husband and wife, and that after Jesus' death, the Magdalene settled in the south of France and sired a lineage which persists up to this day, and which has been protected by a variety of secret societies, including the Knights Templar and the Priory of Zion.
Though Brown's book recites this theory, he gives full credit to Leigh and Baigent. Furthermore, it forms a small portion of the "completed work." The characters, plot, events, settings and textual conflicts in the novel go far, far beyond Leigh and Baigent's work. The Da Vinci Code stands on its own as a work of fiction. Certainly Dan Brown was cognizant of Holy Blood, Holy Grail, he mentions it in his novel. But ideas, in and of themselves, are not copyrightable. It is within my rights as an author to cite other authors' work for analysis, discussion and/or inspiration. Having read both works, the conclusion I have reached is that Brown's best ideas are his own, that his use of the theories presented in Holy Blood, Holy Grail is innocuous and does not rise to the level of plagarism. Having CITED the work in question it cannot be plagarism in that word's common definition and meaning.
Let's also look at Leigh and Baigent's motives. There have been two other works which have copied far greater portions of Holy Blood, Holy Grail than The Da Vinci Code. One of them is Foucault's Pendulum by Umberto Eco. The other is an episode of The Secret Life of Jules Verne. Neither of those two works, to my knowledge, were sued by Mssrs. Leigh and Baigent. It was only Dan Brown, with his 40 million copies world-wide, who attracted the lightning-bolts of their solicitors. Where deep-pockets are concerned, you'll always find blood-suckers.
Come back soon for my analysis of the Larry Cohen/Fox lawsuit over The League of Extraordinary Gentlemen, it will answer the question why Alan Moore refuses to have his name associated with V for Vendetta. Don't miss it!