The properly frightened friend, in this latest outing with the Baker Street Irregulars, is Andrew Tillett's schoolmate Benedict Cortland III--whose father has recently drowned, whose grandfather has just had a stroke. ""If I don't come back at the end of the holiday,"" he says to Andrew, ""will you look into it?"" Cortland knows of Andrew's link to Inspector Peter Wyatt, of Scotland Yard; and Inspector Wyatt soon hears of Cortland. Less immediately involved is Third Irregular Sara, about to appear in a play with Andrew's actress mother--but lots of intriguing on-stage and off-stage action will ensue. At Cortland's, his stepmother is chilly, his paralyzed grandfather obviously troubled, eager to answer Andrew's one-blink/two-blink questions. A specialist, who bluffs his way in at Wyatt's bidding, doubts that he's had a stroke; through a fire ruse, another Wyatt emissary manages his removal to a hospital. Meanwhile, Andrew and Wyatt have been stopping in at the Admiralty: Benedict Cortland II had been a naval aide, in Germany, at his sudden death; Benedict Cortland I, aware that his son was onto something, had just returned from investigating the death. . . . It all winds up, dramatically enough, on stage. And, along with lots of atmospheric old-London detail, there's a little leftover business involving big-time Picadilly pickpockets. Stylish and deft, as usual.