It was April in Rome, and gathered together in the church of San Tommaso in Pallario was the kind of varied group of people that can only meet on a tour. They were there under the aegis of one Sebastian Mailer, who had promised them a most unconventional tour — a claim no one later disputed, after encountering murder, blackmail and drug-running. Inspector Roderick Alleyn, in Rome on a special mission, became involved in the case, and found it one of his most baffling — a case in which every suspect might equally well prove a victim…