‘The Americans fingerprint everyone when they pass through immigration – part of the tightened security after 9/11, right?’

I shrugged. ‘OK.’

Greene held up his hands like it was a conjuring trick. ‘And guess who doesn’t have fingerprints any more! They put me on the first flight home!’

I stared at Billy Greene’s blackened hands and I kept on staring until he placed them on his lap, below the table, where I could no longer see them.

‘Harry Jackson,’ said Sergeant John Caine. ‘People tend to walk right past old Harry Jackson.’

It was true.

The Black Museum was so full of gory artefacts, from the endless variety of firearms and blades that had claimed policemen’s lives to the pots and pans in which serial killers had cooked the flesh of their victims, that it was easy to walk past the small, unassuming display devoted to Harry Jackson.

There wasn’t much. A glass frame containing a short newspaper clipping, yellow with age. Two typewritten paragraphs of explanation.

And the whorls of a dead man’s thumbprint.

‘Harry Jackson was the first man to be convicted in England because of fingerprint evidence,’ Sergeant Caine said. ‘Harry was a burglar. In the summer of 1902 he climbed through a window in Denmark Hill, stole some billiard balls and left his thumbprint in wet paint on the windowsill.’ Sergeant Caine chuckled. ‘Silly bugger. He got seven years.’

I peered closer at the newspaper clipping. It was a letter to The Times, signed by someone calling himself A Disgusted Magistrate.

Sir, Scotland Yard, once known as the world’s finest police organisation, will be the laughing stock of Europe if it insists on trying to trace criminals by the odd ridges on their skins.

‘Eight years later, the rest of the world was catching up,’ said Caine. ‘Thomas Jennings of Chicago became the first person in America to be convicted by fingerprint evidence.’

‘Another burglar?’

‘No – Jennings was a murderer. And soon the French were at it, nicking a reprobate by the name of Vincenzo Perugia – doesn’t sound very French, does he? – on the basis of a left thumbprint.’

‘What had he done?’

‘He stole the Mona Lisa from the Louvre. Took the Frogs two years to find him because they only had his right thumbprint on file. I hear you solved your case. DCI Mallory would be very happy.’

I nodded, staring at Harry Jackson’s thumbprint.

‘Twenty years ago, a girl called Anya Bauer was the victim of a gang rape at Potter’s Field,’ I said. ‘The Head Master, Peregrine Waugh, killed her to stop her talking. Bob the Butcher – Ian Peck – cut the throats of the men who had been the boys who attacked Anya. We got two murderers. We got their motives. The pieces all fit. The trouble is, John, when you look at them, those pieces don’t make much sense.’

‘Why not?’