‘Someone’s been here in the last half-hour,’ said Claude, as Rocco walked back downstairs. The footprints near the back door were still glistening, and it was clear that whoever had been here had progressed no further before turning and going back out.

Rocco bent and checked the lock, a simple tumbler mechanism. There was no sign of the door having been forced, and the tongue was shiny and well oiled.

‘Whoever it was had a key.’ He pulled the door shut behind them and said, ‘Could be a cleaner.’

Claude shrugged. ‘Not a local. I’d know otherwise. And why leave the place unlocked?’

‘Maybe we scared them off. Anywhere else we should look?’

‘Right here? Only one other place, smaller than this, at the back of the marais – but I just came by there on my way in. It’s a ruin … unused and wide open.’ He smiled. ‘A weekend visitor got drunk and hung himself from the ceiling a few years ago; the local kids believe it’s been haunted ever since and give it a wide berth.’ He gestured back at the lodge. ‘I was about to check this one when I heard your car. Some sounds carry through these trees.’

Rocco led the way back towards his car. It all seemed perfectly normal. A small village, miles off the beaten track; a couple of lodges used for weekend parties of hunters and fishers. No rush, no fuss, no noise. What could be more innocent? It seemed odd that the owner or owners didn’t use someone from the village to clean for them, but it was hardly illegal.

He noticed a small wooden jetty that he’d missed on his way to the second house. It jutted out onto the larger lake, sandwiched and almost hidden from view by the reeds. He veered off and stepped onto the planks, testing his weight first. He had no wish to go for an unscheduled swim, and no desire to drive back to the house in waterlogged clothing. But water had always held a strange fascination for him.

‘Watch yourself,’ called Claude. ‘You step off there, you’ll go straight down.’

Rocco peered down into a murky brown sludge dotted with lily pads. The atmosphere here was heavy with the smell of rotting vegetation, and out on the water the air was thick with the swirl of flies and midges. Moorhens and coots, startled by his appearance, scuttled away protesting into cover, while a kingfisher flicked past and disappeared into the trees.

‘A few years ago,’ said Claude, joining him with care, the jetty quivering under their combined weight, ‘a kid on holiday jumped off one of the jetties here. Not this one, though. Nice day, warm weather, must have been the most natural thing in the world to go for a swim.’

Rocco waited for the punch line.

‘His body came up three weeks later. It’s like soft toffee down there, waiting to grab you.’ Claude shivered and walked back. ‘Still gives me the creeps when I think about it – and I have to come out here regularly.’

‘Not nice.’

‘No. You ever used that in anger?’ Claude was looking down at Rocco’s gun. He’d forgotten he was still holding it.

‘A couple of times.’ He checked the safety and sighted on a log thirty metres away in the middle of the lake. He hadn’t been to the police range for a few weeks, although regulations required all officers to put in regular practice and submit score cards. Somehow he never found the time. He assumed the position and breathed in. ‘Firing.’

The first shot smacked out and lifted a splash of water two metres beyond the log. It caused pandemonium in the trees, as a score of birds lifted in panic and streaked away into the sky, protesting loudly. The echoes of the gunshot followed them through the marais, followed by another two as Rocco pulled the trigger in quick succession. Another splash with the second shot, then the log exploded as the third one took the damp wood in the centre.

He waved away a veil of gun smoke and looked at Claude. The garde champêtre was staring at him, mouth open.

‘Sorry,’ Rocco said. ‘Looks like I need the practice.’

‘You reckon?’ Claude shook his head and stepped off the jetty. ‘Didn’t look that way to me.’

They returned to the Citroën and Claude bummed a lift back to the village. Just before they reached the end of the track, he tapped Rocco’s arm and pointed off to one side. ‘Stop here a moment. There’s something I want to show you.’

Rocco stopped the car and they climbed out. He followed Claude for fifty metres into the trees, where they emerged into a small clearing, the middle of which was taken up by a circular stretch of water approximately ten metres across.

‘It’s called the Blue Pool.’ Claude pointed into the water. ‘Take a look.’

Rocco stepped up to the edge of the water and felt the hairs on his neck stand up.

The water was crystal clear all the way to the bottom, and about the same depth as the deep end in a public swimming pool. It was also the same colour blue, and the sides were uniformly curved, like a giant soup bowl.

‘I don’t get it,’ said Rocco. He’d never seen anything like it. And after the impenetrable murk of the lake just a short distance away, it was a distinctly odd contrast.

‘Creepy, isn’t it?’ said Claude. He bent down and dug his fingers into the side of the pool just below the waterline. When he brought his hand back up, his fingers were covered in soft chalk, like cream cheese. ‘It’s something to do with the chalk and the chemicals in the soil. It never gets dirty except after a storm, when mud gets washed in, and always stays the same depth. And nobody ever goes swimming in here.’

‘Not even kids?’

‘Especially not kids.’ He pointed to the bottom. ‘See that small dark area right in the middle?’ He stood up and cast around until he found a short branch, heavy with mud. He dropped it into the water and hunkered down to watch. Within seconds, the branch, too heavy to float, began to slide down the curved side of the pool until it reached the centre.

Then it was gone.

Rocco couldn’t help it; he stepped back from the water’s edge with a start. ‘What the hell happened?’

Claude shrugged. ‘I think it’s a freshwater spring, like a fumarole. Anything near the neck of the inlet gets sucked down by some kind of back pressure.’ He stood up and pulled a face. ‘Actually, I don’t have a clue how it works, but that’s what a water authority inspector told me a while back.’ He bent and scooped up a handful of water and tasted it. ‘Try it. It’s as good as Evian.’

Rocco shook his head. ‘I’ll take your word for it.’ He tried not to think about what else might have got dragged down there over the years and left its traces behind. No wonder nobody swam in there: the very idea would give even strong men the jitters.

As they walked back to the car, he looked back at the first lodge, silent and anonymous, shuttered against prying eyes.

‘I’d like to take a look inside,’ he said quietly. ‘What are the chances?’

Claude stopped and pursed his lips. ‘I’ll see if anyone knows who owns it.’ He dipped a hand in his pocket and took out a slip of paper. ‘Damn – I forgot. I had a phone call this morning. Wouldn’t give me his name, said you wanted this urgently but he wants to be left out of it. He sounded a bit shifty.’

Rizzotti. Rocco studied the names Claude had transcribed onto the back of an old fishing permit. The first was the senior magistrate who had signed the release papers for the dead woman. The second was the name of the dead woman herself, followed by an address. He felt his gut tighten. It was near the Bois de Boulogne, an area he knew well. Big houses, expensive cars, entryphones on the gates and armed guards for those who found that kind of accessory a necessary part of life. Not the kind of place you went calling unless you had a solid reason for being there.

‘She’s not just anyone,’ said Claude. It wasn’t a question – he’d written down the name and evidently recognised it.