We stepped into the dusty room and I looked around in disbelief. There was a stovepipe hat, half eaten away by moths and mould. Cardboard boxes overflowing with old-fashioned rubber truncheons. A collapsed stack of elderly riot shields. Metropolitan Police baseball caps that had never quite caught on. A rack of heavyweight Kevlar jackets, nothing like the wafer-thin stab-proof numbers that we had these days. And there were other bits and pieces of uniform – helmets with the badge gone, jackets with their brass buttons missing, abandoned kit that had to be twenty, fifty, a hundred years old. Police junk that nobody quite had the heart or the energy or the permission to chuck away.

So they stuck it in here.

‘Have you heard of the Black Museum?’ Greene said. ‘It’s in New Scotland Yard. It’s closed to the public. This is just like the Black Museum.’

I smiled. ‘This is a bit more of a mess than the Black Museum. It’s true that the Black Museum has a lot of old kit. Guns. Knives. Walking sticks that turn into swords. Umbrellas that turn into guns. They’ve got a sword that turns into a knife called a Cop Killer. But the Black Museum is not really a museum at all. It’s more of a classroom. A training aid.’

Greene’s eyes were wide. ‘You’ve seen it?’

I nodded. ‘Part of my training. They’ve got a display about Met officers killed in the line of duty. They show you around the Black Museum so that it doesn’t happen to you.’

Greene took a deep breath and let it out slowly as he turned to point at a dusty shelf in the darkest corner of that strange little room.

‘Look,’ he said.

‘I can’t see anything,’ I said, taking a step closer to where he was indicating.

And then I saw it.

It was an ancient leather bag sitting all by itself on a shelf tangled with cobwebs. The dark brown cowhide was worn and cracked. The brass hardware and locks were blackened with rust. Greene lifted it up for my inspection and at the back of the shelf a spider scuttled away as if late for an urgent appointment.

‘It looks like an old doctor’s bag,’ I said.

‘It’s a Gladstone bag,’ Greene said. ‘What makes this one special is that it’s a Murder Bag. I think it’s one of the originals. Did you ever hear of Murder Bags?’

I shook my head.