DCI Forsyth amd DS MacRae are called to a house from which a young lady has been kidnapped and where they find the sister and brother of the victim at loggerheads about the involvememt of the police. The first attempt to pay the ransom is aborted when the bug hidden in the money is found to have been removed by one of a small number of possible suspects. At the second attempt, the room in which the ransom is to be handed over is under surveillance by MacRae. He sees the brother go in but no-one comes out. When he goes to investigate, he finds the brother stabbed to death and the money gone. The squad attempts to find a solution to the impossible crime but without success and it is left to Forsyth to arrive at an explanation of the mystery and the identity of the kidnappers by the use of logical reasoning.