With fewer Egads and zesty Britishisms than is his wont, and fewer appearances by his castle roommate, 400-year-old ghost Sir Roger de Rudisell, ""retired"" spy Hannibal Sterns applies his spoofy sleuthing to the double disappearance of Crown Prince Brulph and--more serious yet--his crown. (As the Archbishop points out when the prince disappears, ""the only thing really needed for a coronation is a prince--Brulph's cousin Basil, or Blodwign, will do just as well--and of course the crown! . . . It would be a pity to waste all the food and flags and fun."") And of course with Byfield pulling the strings, the ghostly hand and screaming ravens and mob assault on Hannibal's castle are all just part of the festivities. Without the heavy fog--which looks, drolly, like inky pools from which emerge the bits of action that Byfield wants you to see--there would be no mystery; as it is, her down-to-the-last-detail explanation involves nothing more sinister than a royal hangover. Despite the diluted diction, a sufficiently sophisticated caper for Hannibal's alert audience.