It was late afternoon by the time Diane Fry arrived at Ringham, and she was in a bad mood. She drove up the track past the farmhouse to where she could see other vehicles parked on the hill.

‘Where is she?’ she asked when she saw Cooper and Weenink under the trees. Cooper bent down to her car window.

‘She’s up there.’ He gestured vaguely, irritating her still further.

‘On the moor?’

Fry got out of the car and flexed her leg. She could feel her knee starting to swell up. She ought to be at home with a bag of frozen peas on it – if only she had any frozen peas in the freezer compartment of her fridge. She struggled up the rocky slope to look towards the plateau. She was no more than half a mile from Top Quarry.

‘Where is she exactly?’

‘Near the Cat Stones, where she was attacked,’ said Cooper. ‘It was Mark Roper who reported sighting her earlier this afternoon. She refuses to come down. We were just discussing taking her into custody for her own safety.’

‘What?’

‘She can’t stay up there. She’s not safe. What if she runs into our killer? That would be just great, wouldn’t it?’

‘It’s not very likely.’

Cooper shook his head in exasperation as she pulled on her black jacket. ‘OK, I’ll come with you.’

‘Don’t bother,’ she said.

Fry began to walk away, tugging her jacket around her as she strode towards the path, brushing past a PC talking on his personal radio. Cooper and Weenink watched her go. Weenink’s expression was puzzled as he leaned towards his partner.

‘Ben?’

‘Fetch the car,’ said Cooper.

‘Where are we going?’

‘To see Mark Roper again.’

‘Why?’

‘Because I feel like something to cheer me up.’

‘But –’

Cooper gritted his teeth. ‘Will you just fetch the car?’

‘Jesus,’ said Weenink. ‘I thought it was only women who had a wrong time of the month.’

Mark was sitting on the ground in his red fleece jacket, with Owen’s walling hammer in his hand. Occasionally, he dug the cutting edge of the hammer hard into the soil and studied the shape of the gouge he had made.

‘So is the wall finished?’ asked Cooper.

‘I thought it was,’ said Mark. ‘But look at that.’

He pointed down the length of the newly-rebuilt stretch. The stones had bulged and bellied outwards, and the coping stones had slipped from their places, exposing the filling, which trickled from the interior of the wall like grain from a split sack.

‘What did that?’

‘A rotten stone,’ said Mark. ‘One single rotten stone that crumbled with the weight and let down everything above it. Owen must not have spotted it when he put it in place. He says every stone has to play its part. You can’t have weaknesses, or the whole thing comes down.’