Five years after giving up his life of crime in the sewers Montmorency is back in London. But his evil alter ego, Scarper, is back too, pulling him into a dark world of addiction. His old friend and fellow government agent, Lord George Fox-Selwyn, fights to rescue him from disaster, and risks calling in Montmorency's prison doctor, Robert Farcett, to help. However, what should be a rest on a Scottish island throws up another mystery, with the discovery of a row of tiny graves . . .
As Montmorency struggles to regain his health, he is called back to London, where unexplained bomb blasts have reduced the government to despair. Can he and Fox-Selwyn catch the bombers? How far can they bend the rules and still stay on the side of good? Can Doctor Farcett find out why the babies have died? And will Montmorency be able to return to life at the Marimion hotel without falling back into his old ways?
