‘But what does all that mean?’ asked Lesley.

I took a pull on my Becks and waved the bottle at the TV. ‘It means that I’ve just figured out how the fire was started.’

The next morning Lesley emailed me the fire report, and after I’d checked that I tracked down a retail equipment store that could deliver a till just like the one used in J. Sheekey’s Oyster Bar. Because of Nightingale’s No Visitors in the Folly, Not Counting the Coach House rule, I had to carry the bloody thing from the tradesman’s entrance down into my lab all by myself. Molly watched me staggering past and covered her smile with her hand. I figured Lesley didn’t count as a visitor in this instance, but when I called and invited her over for the demonstration she said she was busy running errands for Seawoll. Once I had everything in position I asked Molly to ask Nightingale to meet me in the lab.

I cleared an area in the corner, away from any gas pipes, mounted the till on a metal trolley and plugged it in. When Nightingale arrived I handed him a lab coat and eye protectors and asked him to stand on a mark six metres from the till. Then, before I did anything else, I removed the battery from my mobile phone.

‘And the purpose of this is what, exactly?’ asked Nightingale.

‘If you’ll just bear with me sir,’ I said, ‘It’ll all become clear.’

‘If you say so Peter,’ he said, and folded his arms. ‘Should I be wearing a helmet as well?’

‘That’s probably not necessary, sir,’ I said. ‘I’m going to count down from three, and on zero I’d like you to do the strongest magic consistent with safety.’

‘The strongest?’ asked Nightingale. ‘You’re sure about this?’

‘Yes sir,’ I said. ‘Ready?’

‘Ready when you are.’

I counted down, and on zero Nightingale blew up the lab – at least, that’s what it felt like. A ball of burning fire, like a werelight spell gone horribly wrong, formed over Nightingale’s outstretched palm. A wave of heat washed over me and I smelled crisping hair. I almost threw myself behind a bench before I realised that the heat wasn’t physical. It couldn’t have been, or Nightingale would have caught fire. Somehow the heat was all contained within the sphere above his hand – what I’d felt was vestigia on a grand scale.

Nightingale looked at me and calmly raised an eyebrow. ‘How long do you want me to keep this up?’

‘I don’t know,’ I said. ‘How long can you keep it up?’

Nightingale laughed. I caught a flicker of movement in my peripheral vision and I turned to find Molly standing in the doorway, eyes shining with reflected fire and fixed on Nightingale.