Twenty-ninth in the long-running mystery series with Dr Launcelot Priestley.

The old man dragged his dilapidated chair to the window. With difficulty, he slowly extended a gnarled, shaking hand and pointed toward a distant, formless bulk outlined against the sunset. "The tower still stands," he said in a high-pitched, quivering voice, which seemed to conceal a note of triumph.

Strange words from a man who has just been told that his eldest son lies dead, killed by the inescapable explosion of his own shotgun.

To be sure, the body had been found near the tower, but what could be the significance of this ungainly structure that the old man should mention it so mysteriously? Could the key exist within the old letter bearing biblical citations alongside a cipher of odd, hand-drawn shapes?

Subsequent developments draw Jimmy Waghorn and Inspector Hanslet far from the actual crime scene in their search for the murderer. When they finally bring their theory to that intrepid scientist-detective, Dr. Priestley, he offers a strangely enigmatic suggestion which throws new light on the case and sets them on the track of an amazing discovery.