The three young Carstairs are definitely new characters in the mystery novel line-up of detectives. To be sure they considered they were acting as a front for their adored and irresponsible mother, a charming widow who supported her trio by writing mysteries. They used her technique -- even her lingo -- when murder happened under -- almost -- their very eyes, next door. They invented evidence, and destroyed clues, and interviewed suspects, and made it very difficult for the police. But they brought off a coup -- and lined up a prospective step-father.