‘That’s it?’ I demanded.

‘Yes. Everything is here!’ He didn’t notice the dire disappointment in my voice. Or if he did, he chose to completely ignore it. His dark eyes were glittering with an inner frost, as if he had just been given an award by the International Miser Society.

With silent reverence, he held up the envelope for a moment, as if it indeed were the Holy Grail to him. Maybe it was. Then he slipped it into his pocket, and from his other one withdrew a similar-looking envelope, which he placed inside the safe before closing and locking it.

What was that about? Why not just take the envelope? Why leave one behind? Was it an apology letter? Sir, I am deeply regretful to have had to disturb your criminal operation, but it was necessary to retrieve an item which you stole from me. My sincerest apologies, Rikkard Ambrose.

I glanced at Mr Ambrose’s chiselled face and shook my head. No. He wouldn’t write anything remotely like an apology, or write or say anything at all for that matter. He would just stay silent, in the knowledge that he had given his opponent a solid figurative kick in the bollocks. So what was the envelope for?

I burned to ask, but this was neither the time nor place. We had to-

‘We have to get out of here,’ Mr Ambrose cut short the very same words in my mind. He sprang to his feet and strode over to the door. Carefully, he peeked outside. ‘The guards are still nowhere in sight. If we hurry, we can reach the tunnel before they arrive and the alarm is raised.’

He was already about to open the door when, suddenly, an idea struck me and I grabbed his arm.

‘But why leave at all?’ I demanded.

Turning, he threw me a look that could have frozen lava. ‘Would you prefer to stay and ask for hospitality? I imagine Lord Dalgliesh would be delighted to receive you for tea and biscuits. Especially when you will have such interesting topics of conversation as where the most precious document on this entire island has disappeared to.’

‘I meant,’ I said, trying to be patient, ‘why should we run now, before the guards arrive? We could shut the door of the hut and stand outside like real guards until the next shift arrives. They will think we are the real guards, the ones they’re supposed to be relieving, and we'll saunter off without anybody ever being the wiser.’

It may have been only a trick of the torchlight, but I thought I saw Mr Ambrose’s mouth drop open slightly. He was quiet for one or two moments. Then he said:

‘This… actually sounds as if it were a reasonably feasible plan.’

‘Blimey! Don’t be all over with me with your compliments!’

‘Don’t worry. I won’t.’

‘So we’re going to do it?’

He hesitated. I could see the struggle in his eyes - the same struggle as on the day I had asked for a dress and a bag of onions. He hated to adopt any plan of mine, probably because it meant admitting I actually was of some use. But he was nothing if not practical, and - I could see the thought enter his mind as clearly as if it were painted on his forehead - at least this plan wouldn’t be expensive.

‘All right,’ he conceded. ‘We will.’ And he stepped outside to take up his position beside the door.

To my own great surprise, my plan actually worked perfectly. The two guards showed up only two minutes after we had left the hut, greeted us in a quite friendly manner and sent us off downstairs. I followed Mr Ambrose down at a steady pace, although what I actually wanted to do was run.

Stay calm, I told myself. There is no need to run. Nobody knows the file is missing. You can walk out of here slowly and nobody will ever know. Everything is going great.

Yes, everything was going great - until, as we passed under a shadowy arch of stone, I saw, a few dozen yards away, the two guards we had relieved of their duty half an hour ago. They were engaged in an energetic discussion with Colonel Townsend.

‘I? Send them up there?’ the colonel was saying. ‘No, why in God’s name should I do something like that. I thought they were the regular shift that…’

Mr Ambrose had seen them, too. He stiffened.

‘Seems like not attracting attention is no longer an option,’ he stated icily. ‘Move. Now!’

Grasping my hand, he tugged me away from the colonel, towards the entrance of the tunnel he had pointed out earlier. He didn’t have to tug hard. I hurried after him, trying my best to keep up with his long strides. He was right. We had to get out of here right now, or we were as good as dead. Quickly, we neared the entrance to the tunnel. There was a soldier standing beside it. A guard?

‘Do you think he’ll try and stop us from entering the tunnel?’ I asked out of the corner of my mouth, nodding towards the soldier.

‘It is interesting how you always seem to assume that I know everything about this place, when, in fact, I haven’t been aware of its existence any longer than you have, Mr Linton. I have no idea.’

‘Well, what if he does?’

No answer.

‘Sir?’

Silence. So I just continued on, trying to ignore the rising feeling of panic in my stomach. The guard definitely looked alert and suspicious enough to justify my fears. He had a narrow rat’s face, with a long, twitching nose. I had never trusted people with long noses.

‘Sir?’ My voice was a harsh whisper. ‘Mr Ambrose, Sir, what will we do if he doesn't let us pass? Sir?’

More silence. I looked up at his face and saw that, although it was cool and serene as ever, his eyes were totally focused on the guard, burning with cold ice. Maybe, just maybe, he didn’t know what to do yet, either.

We were only ten yards or so away from the tunnel entrance now. I tried to look as innocent as possible.

If you think about it, we are innocent, right? After all, we’re just stealing back something that had been previously purloined.

One hundred per cent correct. My ears, though, didn’t seem to agree: they were red hot with guilt. Never before had I been so thankful for my tanned skin, which would at least hide the blush on my cheeks.

Five yards.

Four.

The guard didn’t move.

Three yards.

Without warning, the guard stepped sideways, blocking our way. My hands clenched into fists, and it took a conscious effort to relax them, and to look the man straight into his little rat’s eyes.

‘Hey, you there! You know nobody is allowed in the tunnel without permission from the colonel.’