He stopped, looked at the screen again, and remembered the call he’d tried to make to Freddy Robertson. The professor wasn’t at home tonight.

‘Oh, shit,’ said Cooper. ‘He’s gone there now.’

Freddy Robertson’s BMW was missing from the drive in front of his house, and there was no answer to the door.

‘OK, let’s get it open,’ said Hitchens. ‘Not too much damage, if you can help it.’

Fry watched the oak door being forced. She didn’t really mind if it was damaged. In fact, she rather hoped that the mosaic tiles in the hall might get cracked and the mahogany balustrades chipped. Accidentally, of course.

She followed the team into the house as they checked the rooms to make sure no one was inside. She was looking for a cellar, which she felt sure must exist. An image of the crypt at Alder Hall was strong in her mind – the innocuous door off the hallway, the stone steps down into darkness, the smell of damp and earth.

At first she could see nothing, and she began to think she was mistaken. But finally Fry realized she was looking for the wrong thing. She put Alder Hall out of her mind, walked into the kitchen and lifted the edge of a rug laid over the tiles. And there was the trap door.

She called for assistance to roll back the rug, then unfolded the brass ring set into the wood. The hinges worked smoothly, though the door was solid and heavy. When it was fully open, wooden stairs were visible below floor level. She couldn’t quite identify the smell that rose from the opening. Not damp and earthy, as she’d been imagining, but something sweet. Sweet and slightly sickly.

Fry looked around. But this time she didn’t need to ask. Lights were already being brought. Plenty of lights.

This time, Cooper found no one watching him from the doorway of Greenshaw Lodge. The place was in darkness, and when he drew up near the steps, his headlights showed that the back door stood open.

Taking his torch from the glove compartment, he banged on the front door and rang the bell. Then he followed the path to the back door and knocked on the glass panel. He could see the gleam of white shapes in the kitchen – fridge, cooker, washing machine. But no glimmer of light any further into the house.

‘Hello? It’s Detective Constable Cooper. Anybody home? Mr Slack?’

There was no response. The Slacks didn’t have a dog, so there wasn’t any barking, as there might have been at Tom Jarvis’s place.