Dickson's Arthurian series about a dragon knight--twentieth-century mathematician Jim Eckert, who has been thrown back to the fourteenth century and has acquired the ability to turn into a dragon at will--continues. This time Eckert discovers that Lyonesse, the refuge of King Arthur and his knights, is being menaced by a dark power. Arthur and the knights are reluctant to accept the aid of a dragon with magical powers and even more reluctant to draw on twentieth-century knowledge for help. Eckert has to be diplomat, warrior, and puzzle solver all at once, then, and quickly. Dickson's is a distinctly original take on the Matter of Britain (which, to be sure, is supposed to have transpired a good many centuries earlier than in these yarns), distinguished by the humor arising out of the contrast between popular notions about the Middle Ages and its frequently grisly realities.