‘This,’ he said, ‘is the sacred part.’ His hushed voice sounded hollow, like the echo round an indoor swimming pool. Intrigued, I braced one hand against the paper-cool stone and leaned forward for a proper look.

The water was there, as he had promised. Clear, holy water, pale turquoise in the glare of an electric light that hung from the stone arch overhead. It was, Paul told me, a Merovingian well, meaning it dated back to the time of the Franks – older, he thought, than Sainte Radegonde herself. The shaft sank deep and straight and true; several metres deep, I would have said, and yet the water was so amazingly clear that I could see the scattering of pebbles at the bottom.

‘You can even see the footholds,’ Paul said, pointing, ‘that the well-diggers used to climb out again, after they’d struck water.’ The footholds ran like a makeshift ladder, straight to the bottom of the well – small, even squares the width of one man’s boot, gouged in the yielding yellow stone.

I sighed, and the surface of the water shivered. ‘Well,’ I said, ‘at least we know that Harry isn’t anywhere down here.’

‘A cheerful thought,’ Paul smiled. ‘But you’re right, it’d be pretty hard to hide something in that water. Even a King John coin.’ He flipped a tiny half-franc piece into the well to demonstrate, and we watched until it came to rest upon the bottom, clearly visible. ‘Here, make a wish,’ Paul told me, handing me another coin.

He sounded just like my father, when he said that. For a moment I was five years old again and standing at the rim of the fountain in the courtyard of our house in Italy. But then I caught my own reflection in the water of the well, and the child vanished. I shook my head. ‘I don’t have anything to wish for.’

‘Everyone has something to wish for. Besides, how often do you get a chance to make a wish in holy water?’

‘No, honestly, you needn’t waste your money …’

‘Has anyone ever told you you’re a terrible cynic?’ He grinned and closed his eyes. ‘OK, never mind. I’ll make a wish for you. There,’ he said, and tossed the second coin into the waiting well. It hit the water with a satisfying plunk. End over end my unknown wish tumbled, glittering, and landed close to Paul’s one on the smooth and level bottom.

‘So what did I wish for?’ I asked, curious.

‘Bad luck to tell,’ he reminded me. Turning, he offered me his hand to help me up the stairs again. ‘Right,’ he said, ‘let’s give this place a proper search, and see what we can find.’

We found a treasure trove of slightly dusty artefacts displayed in every crevice of the caves. We found a smaller chamber at the tunnel’s end, where someone evidently lived from time to time. ‘There’s a caretaker in the summer,’ Paul informed me, ‘on and off. I think she stays up here.’ But no one had been staying here recently. The bed was stripped, the cupboards empty, and dust lay thickly settled on the floor, marked by no sign of footprints save our own. We found a working wine press tucked in one high corner of the largest cave.

What we didn’t find, of course, was anything that Harry might have left. There wasn’t so much as a chewing-gum wrapper or a shred of tissue dropped in that bright, winding underground maze. I was, by turns, relieved and disappointed. Relieved because I hadn’t turned up any evidence that Harry was in trouble, disappointed because I hadn’t turned up any evidence that Harry was here at all. There was only that blasted coin.

Paul poked at the donation saucer as we paused before the simple altar on our way back out. ‘Is this where you found the King John coin?’

My face flamed with embarrassment, but I didn’t bother to deny it. The problem with Paul, I thought, was that he was too damn clever. He had a quiet but persistent way of finding out the truth. ‘Yes. I … I put in a donation of my own,’ I added, as if that made my theft acceptable, but Paul didn’t seem to be listening.

‘There’s got to be an explanation.’ That was the physicist talking. He furrowed his brow and stared hard at the plate of jumbled coins. ‘There’s got to be. We just aren’t looking at this from the right angle.’

He was still standing there, thinking, when the faint sound of the noonday bells came drifting up from the town below and broke the peaceful silence of the chapelle. There was nothing more for us to do here, I decided. I tugged at Paul’s sleeve. ‘Come on, Sherlock, time for lunch.’

‘Yeah, OK.’ He glanced at his watch. ‘I guess I ought to check the laundry, anyway, before Simon gets back. Thierry’s probably shrunk everything beyond recognition by now.’

His gloomy fears turned out to be unfounded. From the pristine pile of folded shirts and jeans that met us in the hotel’s entrance lobby, it appeared that Thierry had done quite an expert job.

He flashed his quick disarming smile and ran his thumb along a trouser crease. ‘I cannot take the credit,’ he confessed. ‘I gave the clothes to Gabrielle for washing.’

Paul raised his eyebrows. ‘Gabrielle?’

‘The girl who does reception this week. Me, I am not good at washing things.’

He’d never have to worry about it, I thought, as long as he could aim a smile like that at a member of the opposite sex. It was a difficult smile to resist. I couldn’t help but feel a pang of sympathy for Gabrielle – small wonder she was so confused, sometimes. ‘You don’t play fair,’ I said to Thierry.