Dr Walid produced a covered stainless-steel tray. ‘I knew you’d ask that,’ he said, ‘so here’s something I whipped out earlier.’ He lifted the cover to reveal a human brain. I’m no expert, but it didn’t look like a healthy brain to me; it looked shrunken and pitted, as if it had been left out in the sun to shrivel.

‘As you can see,’ said Dr Walid, ‘there’s extensive degradation of the cerebral cortex and evidence of intracranial bleeding that we might associate with some form of degenerative condition, if Inspector Nightingale and I were not already familiar with the true cause.’

He sliced it in half to show us the interior. It looked like a diseased cauliflower.

‘And this,’ said Dr Walid, ‘is your brain on magic.’

‘Magic does that to your brain?’ I asked. ‘No wonder nobody does it any more.’

‘This is what happens if you overstep your limitations,’ said Nightingale. He turned to Dr Walid. ‘There wasn’t any evidence of practice at his house. No books, no paraphernalia, no vestigium.’

‘Could someone have stolen his magic?’ I asked. ‘Sucked it out of his brain?’

‘That’s very unlikely,’ said Nightingale. ‘It’s almost impossible to steal another man’s magic.’

‘Except at the point of death,’ said Dr Walid.

‘It’s much more likely that our Mr Coopertown did this to himself,’ said Nightingale.

‘Then you’re saying he wasn’t wearing a mask during the first attack?’ I asked.

‘That seems likely,’ said Nightingale.

‘So his face was mashed up on Tuesday,’ I said. ‘Which explains why he looks blotchy on the bus cameras, then he flies to America, stays three nights and comes back here. And all that time his face is essentially destroyed.’

Dr Walid thought it through. ‘That would be consistent with the injuries and the evidence of the beginnings of regrowth around some of the bone fragments.’

‘He must have been in some serious pain,’ I said.

‘Not necessarily,’ said Nightingale. ‘One of the dangers of dissimulo is that it hides the pain. The practitioner can be quite unaware that he’s injuring himself.’

‘But when his face was normal-looking – that was only because the magic was holding it together?’

Dr Walid looked at Nightingale.

‘Yes,’ said Nightingale.

‘When you fall asleep, what happens to the spell?’ I asked.

‘It would probably collapse,’ said Nightingale.

‘But he was so badly damaged that once the spell collapsed his face would fall off. He’d have had to keep the spell up the whole time he was in America.’ I said. ‘Are you telling me he didn’t sleep for four days?’

‘It does seem a bit unlikely,’ said Dr Walid.

‘Do spells work like software?’ I asked.

Nightingale gave me a blank look. Dr Walid came to his rescue. ‘In what way?’ he asked.