In the popular adult author's third juvenile fantasy, a brother and sister slip from the present into the past and expose an evil enchantment--cognizant the while, as the reader will be, that this is like ""something in a storybook."" It's all, indeed, storybook-like. Young Britishers John and Margaret are vacationing with their parents in the Black Forest's Wolfenwald when a weeping man in medieval dress happens by. Leaving their drowsing father (but ""sensibly"" leaving a note), the children follow the unhappy fellow and shortly find a gold chain and portrait medallion engraved OTHO FIDELIS that he's apparently dropped. At a derelict cottage, they spot his discarded clothes and discover. . . ""an enormous wolf,"" who flees even as John hurls the medallion at him. Come morning, the man reappears, and they learn he is Lord Mardian, condemned by the evil Almeric to be a wolf by night--so that Almeric, who has made himself into Mardian's double, can surreptitiously dispatch Mardian's blood-brother Duke Otho (and, presumably, rule in his place). Only the medallion can expose Almeric as a usurper, restore Mardian to Duke Otho's side, and relieve Otho's fatal ""melancholy."" So John and Margaret, now yclept Hans and Gretta, must be sneaked into the castle. . . where Margaret's unmaidenly independence and spirit (""she saw how, in the past, girls really had something to complain about""), combined with John's unmedieval enterprise and kindliness, bring everything off quite neatly. The greatest danger in any such situation--that John and Margaret might somehow lose touch with the present--is briefly suggested, then more or less ignored: at the close, their father is waiting as they leave the castle. . . and it might all have been a dream. It might just as well be, so little substance does Stewart invest it with.