Arthur Bryant, John May and the Peculiar Crimes Unit are on the trail of an enigma: a young man called Mr Fox. But his identity is false, his links to society are invisible and his home yields no clues. All they know is that somehow he escaped from a locked room and murdered one of their best and brightest. Now the detectives are being lured down into the darkest recesses of the London Underground where their quarry, expertly disguised, has struck again.