‘Sorry, no triple espresso in here,’ John Caine told me, holding out a mug of steaming tea. He took a sip from his BEST DAD IN THE WORLD mug.

‘You’re having a bit of a quiet week, aren’t you?’ he said. ‘Nobody’s been hanged on YouTube.’

I nodded. ‘Three murders in July and now nothing for the first seven days in August. Why would they stop, John?’

‘Lots of reasons why they might jack it in. One of them might have died. They could have fallen out with each other. Somebody’s wife found out what her old man has been doing and begged him to stop for the sake of the kiddies. Or – most likely reason a crew stops – they might think that they’ve been rumbled and you’re going to kick down their front doors at five o’clock tomorrow morning.’

I laughed bitterly.

‘Not much chance of that happening.’

DCI Whitestone had not turned up for work this morning because her son was having another operation on his eyes. This meant I was still the acting SIO. In the absence of any leads, I had done what I always do when I need guidance – come for a cup of tea at the Black Museum.

‘Or one of them lost their nerve,’ John said. ‘Or all of them lost their nerve. Or they’ve ticked off everyone on their kill list. That’s possible. Or they’re quitting while they’re ahead because they’re intelligent enough to know that if they keep doing it, they’ll get caught.’ He took a thoughtful sip of his tea. ‘Maybe they’re cashing out while they’re ahead.’

‘You ever meet a villain that smart?’

‘Not yet.’

‘Me neither.’

I inspected the ropes on the hanging tree. The oldest – four thin strands now black with age – was two hundred years old. The newest – in pride of place at the front, like the star on top of a Christmas tree – dated back to 1969, the year that capital punishment was officially abolished. It was made of eight thick strands of hemp that were gathered in a large brass thimble. They looked slick and sticky.

‘Vaseline,’ John said. ‘Stops the rope from burning off skin. That’s the Sixties for you – when they hanged you with a bit of compassion.’

‘What am I doing wrong? I’m no nearer to them than I was a month ago.’

He adjusted the ropes on the hanging tree. If anything was touched in here, then he wanted it to be exactly as it was before.

‘You’re not following the leads you’ve got,’ he said.

‘But I don’t have any leads. No kill site. No witnesses. No prints that are worth a damn – nothing that shows up on IDENT1.’

He looked at me as if I was missing the obvious.