According to the
Priory of Sion in Baigent et. al's
Holy Blood, Holy Grail, Thomas is the
scion of the
Merovingian line of
Frankish kings, who (again, according to Baigent et. al) are the stewards of the bloodline of
Jesus himself.
Little is known of this would-be monarch. He took control of the Priory as Grand Master from his father, Pierre Plantard de Saint-Claire. Whether Pierre released control in 1984 or 1988 is a matter of some debate, although it seems most likely that Thomas took control in 1989. Also a matter of debate, naturally, is his claim to the throne of France. In a 1999 interview, Prince Michael Stewart of Scotland (who knows a little something about claims to royalty) completely dismissed the elder Plantard's claim to royalty, saying not that they were out and out lying, but simply full of wishful thinking. As the elder Plantard died in 2000, it is now up to Thomas to prove Prince Michael wrong.
Thomas is also the focus of a number of Apocalypse researchers' investigations as to who might be likely to rise as the Antichrist. Many of these investigations focus on the so-called Grail Families, and Pierre's death in 2000 makes Thomas "the Millenial Grail King", according to some. The Plantard family is a favorite among Grail Families due to its apparent ties to scripture. Aside from claims to be related to Jesus, the name "Plantard" can be translated to "branch" in French, and many passages in Judeo-Christian texts refer to leaders as a "branch" or "plant" of God. Further, the Plantard family crest features two bears guarding a gate emblazoned with the fleur de lis. The bears are interpreted by some as a euphamism for beasts, as in the Beast of Revelations, with the gate being the gate to Hell, or to the Elder Gods, or whatever, and the fleur de lis representing the Tree of Life from Kabbalistic writings. Whether or not this is accurate remains to be seen.
Read the Holy Blood, Holy Grail node for more info.