Part one opens with a very Gandalf-looking Merlin (played by Gérard Jugnot) at the court of Uther Pendragon, where he helped raise and educate the young Arthur and (sometimes) Guenièvre. Ten years later, a less hirsute Merlin retires to Brocéliande after Arthur has grown up and begins to live with Viviane and her son Lancelot. However, Merlin is called back when Camelot is threatened by Arthur's cousin, Morgane, (herself a powerful magic-wielder, trained by her aged aunt) and her ally the barbarian king Vortigern (!) on the eve (?) of Arthur and Guenièvre's wedding. (Morgane had, apparently, expected to marry Arthur and become queen.) Merlin, Viviane, and Lancelot spend some time at court here, and (I think) the two mages are instrumental in stopping Morgane and Vortigern at this time. However, the villains return in part two, and Merlin is ill-prepared because the title of part one, "L'Enchanteur Désenchanté," refers to the fact that Merlin loses his powers as the years pass and he falls in love with Viviane and becomes a surrogate parent for Lancelot.
Taking place seven years later, part two, "Le Secret de Brocéliande," centers on Merlin's struggles to regain his magic in order to save the realm and restore Guenièvre to flesh after she has been turned, by the returned Morgane, into stone. This section also focuses on a quest for the Grail undertaken by Merlin and a teenaged Lancelot--the prophesied Grail-winner--to save Viviane, who is held by Morgane.
There are lots of images and some plot summary at the series official site (http://www.tf1.fr/merlin/) but no video. The following trailer comes from YouTube:
