‘Well I’m not going to tell him,’ said Lesley. ‘Let your governor tell my governor that the fucking ghost of Mr Punch is knocking people off on his patch.’

‘You think it’s a ghost?’ I asked.

‘How should I know?’ she said. ‘That’s what you magic cops are for.’

The Folly has three libraries: one, I didn’t know about back then, number two was a magical library where the direct treatise on spells, forma and alchemy were kept, all of them written in Latin and so all Greek to me, and number three was the general library on the first floor next to the reading room. The division of labour was clear from the start: Nightingale checked the magic library, and I hit the books in the Queen’s English.

The general library was lined with enough mahogany to reforest the Amazon basin. On one wall the stacks went all the way to the ceiling, and you reached the top shelves by using a ladder that slid along on shining brass rails. A row of beautiful walnut cabinets held the index cards, which were the closest thing the library had to a search engine. I caught a whiff of old cardboard and mildew when I opened the drawers, and it comforted me to think that Molly didn’t go so far as to open them up regularly and clean inside. The cards were arranged by subject, with a master index arranged by title. I started by looking for references to Punch and Judy, but found none. Nightingale had given me another term to search for: revenant. A couple of false passes with the index cards led me to Dr John Polidari’s Meditations on the Matter of Life and Death which, according to the frontispiece, had been published in 1819. The same page had a notation in Latin written in an elegant looping hand: Vincit qui se vincit, August 1821. I wondered what it meant.

According to Polidari, a revenant is an unquiet spirit who returns from the dead to wreak havoc on the living, usually in reprisal for some slight or injustice, real or perceived, that the person suffered during their life.

‘It certainly fits our profile,’ I told Nightingale over lunch – Beef Wellington, boiled potatoes and sautéed parsnips. ‘These little grievances going all postal – it fits Lesley’s idea that the big events have little echoes. ’

‘You think it’s infecting them?’

‘I think it’s a field effect, like radiation or light from a bulb,’ I said. ‘I think the echoes are inside the field, their brains get charged up with negative emotions and off they go.’

‘Wouldn’t more people be affected, in that case?’ asked Nightingale. ‘There were at least ten other people in the cinema foyer, including you and Constable May, and yet only the mother was affected.’