A cozy comic mystery in which 13-year-old Amy Whipple is corralled by the wealthy Wylies to pinch-hit as babysitter. Eight-year-old Malcolm Wylie's mother warns Amy that he's ""blessed with a very active imagination"" and ""loves to spin stories""; when he charges ahead to the park and then claims that he's witnessed a classmate's kidnapping in a black limo, Amy is naturally skeptical. Malcolm's father doesn't believe his story, nor do Amy's father (in the delicate position of working for Mr. Wylie) or the police, who remember Malcolm's previous crime reports. However, one ransom note leads to another, evidence mounts, and, devilishly, Malcolm blackmails Amy into cooperating with his investigation. Before long, the two have baited the suspected kidnappers and are tailing them (at night, by moped) to a hideout where bratty, rich little Jennifer Smith is making her abductors' lives miserable. The kidnappers are cartoonishly dense and argumentative, albeit menacing; but Jennifer and Malcolm are so daring and clever that the rescue -- involving laundry and garbage trucks and Jennifer's stunned chauffeur -- is suspenseful as well as funny. Malcolm learns the lesson of the boy who cried wolf, he and Amy become friends, and Jennifer-the-Jerk -- well, she's lucky anybody bothered to save her. A pleasantly thrilling escapade, with some minor implausibilities and only the threat of violence.