Glen Sayers, alert Sydney agent for Moidor cars, is friendly with Dr Fillister, amateur criminologist, and Detective-Inspector McCurley. McCurley, worried over the mysterious murder of Charles Ruddick, inoffensive lecturer at Sydney University, shot from the top of a railway cutting while travelling by train, can assign no motive for the crime. Even Fillister is completely mystified.

Visiting the famous Killenbong Caves, Sayers joins a party of twelve, including Wendy Farren and her brother Bernie. When they reach the Cave of Sindbad, the lights suddenly fail. A cry in the darkness - the guide brings a torch - it is discovered that Bernie Farren has been stabbed! The wires have been cut. When they are joined and the cave illuminated again, there is no sign of the assassin. McCurley, sent down to take charge of the case, has no reason to suspect any of the party, all strangers to the Farrens, while, as the guide locked that section of the caves when the tourists entered, an unauthorised intruder is out of the question.

That night, Sayers drives Wendy back to her Sydney suburban boarding house. She tells him that until a few days before the tragedy she had not seen her brother for two years. She enters the house; Sayers, about to drive away, has an encounter with a drunken man who asserts that he boards in the same place. Next morning he is astounded to learn that the girl has disappeared. Her bed had not been used, and there was no sign that she had returned home the previous night. Search reveals nothing more significant than a letter signed 'Bernie,' hinting that Wendy and her brother had been involved in some risky enterprise, advising her to 'take the swag and go for her life.'

McCurley believes that counterfeit money is at the bottom of the business...(Publisher's abstract)