Rain had blown in through the doorway, creating a dark patch in the stairwell. Inside, the view upwards was dizzying. Stone steps curled away into the tower, with bare tree trunks zigzagging overhead from wall to wall. Cooper could see both the outer and inner surfaces of the staircase at once, which seemed entirely wrong. His instincts were telling him that it was impossible to walk on stairs that coiled so tightly and rose so steeply.

Standing close to the wall, Cooper took hold of the handrail and began to ascend. Mounting the spiral staircase was like walking up a twisted ribbon, or climbing a strand of DNA. It was a sort of stone helix, cold to the touch and smelling of earth. You had to be careful on these steps, or you could fall right through the spiral and plummet to the base of the tower.

Just before the last turn, the bulkhead lights on the wall ended, and Cooper stopped when he saw daylight from the platform. He jumped when he became aware of Kotsev’s breathing below and behind him on the steps. His mind had been so distracted that he’d forgotten his companion.

‘Georgi, you’d better stay back out of sight. We don’t want to frighten him too much.’

‘Dobre. I’ll be right here, behind you.’

Cooper’s heart was beating harder after his climb. All the way up the tower, he’d been conscious of the narrowness of the steps, and the drop through the spiral. One slip could be disastrous.

Slowly now, he eased himself the last few feet on to the platform, trying not to make any sudden noises. Leaving the stairwell was like emerging into a different world, with light and air and an awareness of the valley all around him – banks of trees whispering in the breeze, the cables hissing as they pulled another string of swaying cars across the river. Lowther was standing nearby, his hands resting on the parapet.

‘Mr Lowther, do you remember me? Detective Constable Cooper.’

Lowther seemed to become aware of him for the first time. He tried to back away, but he was already pressed hard against the parapet and could only scrape slowly around the platform until he was on the eastern side. He stood with the Heights of Abraham behind him, birds swooping through the woods, water dripping from branch to branch, cable cars descending to the base station.

Cooper took a step backwards, trying to judge a safe distance that wouldn’t make Lowther feel under too much pressure. At the same time, he had to find some way to keep the man’s attention on him. At the moment, his concentration seemed to be wandering, his eyes darting around the landscape, distracted by the whir of cables and the voices of people on the ground below.

‘Just take it easy, sir. There’s nothing to worry about.’