Even for a detective chief inspector in Homicide and Serious Crime, visits to the Black Museum were meant to be by appointment only. But the curator in Room 101 – a Sergeant John Caine with thirty years’ service on his face and not a gram of flab on his body – greeted Mallory like an old friend.

‘What can we do you for, sir?’ the keeper of the Black Museum said as they shook hands.

‘We’re looking for a knife, John,’ Mallory said. ‘Or at least some kind of double-edged blade.’ He was opening his briefcase. ‘I figure it has to be less than a sword but more than a knife.’ He removed a file containing a sheaf of photographs and spread them on the curator’s desk. ‘Something that could have done this.’

Sergeant Caine calmly studied half a dozen photographs, copies of the same murder scene and autopsy pictures that were on Mallory’s wall in the Major Incident Room, while I looked around me. The walls were covered with bookshelves and badges from police forces around the world, presumably showing their gratitude for a glimpse inside the Black Museum. I picked up an elderly hardback book from Caine’s desk. There was no dust jacket. Forty Years of Scotland Yard, it said. The Record of a Lifetime’s Service in the Criminal Investigation Department by Frederick Porter Wensley.

‘Don’t touch that,’ Sergeant Caine said, not even looking at me.

I put the book down.

To Mallory he said, ‘These are the Bob the Butcher killings.’

‘We’ve yet to make that connection,’ Mallory said.

‘But you’re treating it as a double homicide, sir?’

Mallory nodded. ‘Same killer, same MO. But I’m not convinced it’s Bob.’

Sergeant Caine looked at me without warmth or welcome. Mallory had warned me that he was wary of strangers. Although wary didn’t quite cover his cold, gimlet-eyed hostility.

‘This is DC Wolfe, the newest member of my MIT.’

I held out my hand but Caine didn’t seem to see it. Happy to remind me that, as a sergeant, even one in uniform, he outranked me.

‘Right,’ he said. ‘Ground rules. No photographs. No touching, unless I say so for the purpose of demonstration. And absolutely nothing I say is for the record. Got it?’

‘Got it, sergeant,’ I said.

‘Good. Then let’s go.’

There was a locked door inside Room 101. Sergeant Caine unlocked it and we went inside. It was a living room from the distant past. There was a fireplace, a bay window, gaslights. It took me a moment to register that although these were false, there were weapons everywhere, and these were very real. A glass case full of firearms. A desk covered with what looked like the results of a sword armistice. A hangman’s noose dangled from the ceiling, which I thought was overdoing it a bit.

‘What was the name of the detective who founded the museum?’ Mallory asked.