They've been given just one week to find a killer they'd caught once before . . .

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. Their search takes them into the vast labyrinth of tunnels, a subterranean world full of legends and ghost stations, which tie the city together. Edging closer to what lies hidden beneath the city - and to the madness that is driving a man to murder - Bryant and May are about to uncover a mystery as bizarre as anything they have ever encountered . . .