Vendetta answered that question. She slipped over the edge of the cliff and started to climb down. James had no choice but to follow. He felt dreadfully exposed on the rock face. If one of the guards were to look over the parapet of the dam, he would easily spot them, so they descended as swiftly as they dared.

Thankfully, they made it safely down the cliff and got among the cover of some bushes. They rested for a while, then crawled through the undergrowth until they came to the edge of a dirt track and James saw a brick building set into the side of the hill where a sleepy guard dozed in a concrete sentry post.

What had Ugo said, all those days ago at dinner? That the water came down this side of the mountain through the turbines and into the aqueduct, which crossed over to the palazzo. The brick building must be the entrance to the turbine rooms where the electricity was generated.

Vendetta nodded towards the guard. James wondered if she meant for them to enter here and reach the aqueduct from the inside. That seemed hugely risky; they had no way of knowing what they would find inside, and there was the problem of how to get past the guard.

James was looking around for another route when he saw that Vendetta had her knife between her teeth and was edging out of the bushes towards the guard.

He grabbed her and hauled her back, then shook his head. There would be no more killing if he could help it, and besides, he had seen something. Leading down the mountain towards the aqueduct was a thick iron pipe made of riveted sections.

He pointed to it and set off, skirting round the edge of the track then darting across it once he was out of sight of the sentry post.

Vendetta reluctantly followed. This was not her plan, but when James showed her how easy it was to slither down the pipe using the joins between each section for footholds, she smiled and joined him.

At the bottom water frothed and gurgled out of a large opening and into the canal in the top of the aqueduct.

The two of them peered across to the other side. The aqueduct was exposed for its whole length but James was confident that he could get over without being seen.

He put down his knapsack and opened it, Vendetta watching him with her gleaming, dark eyes. They went very wide when she saw what he had brought with him and was even more amazed when he slipped on the goggles and put the schnorkel between his teeth. She shrank back, scared for a moment. James laughed and pulled the goggles up on to his forehead.

‘I’m going now,’ he said. ‘You wait for me here.’ He pointed to the ground with both his hands. ‘Aspetta –’

‘No,’ she said walking towards the aqueduct. ‘Vengo con voi.’