He nodded at the jar. 'What shall I do with that stuff? May I destroy it?'

I hesitated, then said, 'Yes. Destroy it now. Pour it in the river.'

He raised his eyebrows. 'Are you sure? We could both be accused of treason.'

'I am sure.'

His face flooded with relief. He gripped my hand fiercely. 'Thank you. You have done right, Matthew, you have done right.'

I WALKED DOWN TO the river and stood on the bank watching the ships unload their cargo. Every week came some new wonder. I wondered whether, one day, a ship might bring something else as terrible and dangerous as Greek Fire here. I thought of St John landing a hundred years ago with his papers and the barrel. He had looked at peace in his grave. I knew now that I could never be at peace if I gave anyone in power the chance of making this thing, no matter what the consequences.

I looked across to the far bank, where I had walked with Lady Honor. The bear pit and bull ring rose high above the houses; I could hear a faint cheering from the bear pit – there must be a baiting on. I wondered if Marchamount had enjoyed his afternoon there. What had happened to him? Part of me felt, like Barak, that the game was played out. But the deadly puzzle still nagged at my mind.

A little way off I saw the tavern where we had met the sailors, the Barbary Turk. I went in. At this hour the place was empty and my footsteps echoed on the boards of the large, dusky drinking room. The giant's thigh bone still hung in its chains. I studied it for a moment, then went over to the serving hatch and ordered a mug of beer from the landlord. He was a burly fellow with the look of an ex-sailor about him. He looked curiously at my good stitched doublet.

'We don't often see gentlemen. You were in here a few nights ago, weren't you, talking to Hal Miller and his friends?'

'Ay. They told me of the time they set their table alight.'

He laughed, resting his arms on the edge of the hatch. 'That was a night. I wish they'd given me some of that stuff – I like novelties.'

'Like the giant's bone?' I nodded towards it.

'Ay, it was washed up just by the wharf here. Twenty years ago, in my father's time. Just appeared in the mud one ebb tide. People went hunting for the rest of the giant, but found no more of him. My father took the bone and hung it up here. Imagine what size the man must have been. But we are told of giants in the Bible, so that must be what it is. Better to have had the whole skeleton, but that one bone's enough to bring people here to look and that's good for trade.'

He would have talked on, but I wanted to be alone and took my beer over to the dark corner where I had sat with Barak that night.

His words, though, kept coming back to me. That one bone's enough to bring people here to look and that's good for trade. I thought of the Gristwoods, working with Toky and Wright and whoever their master was for six months before going to Cromwell, trying to make Greek Fire, hunting out the Polish drink. What a profit they must have anticipated. Profit from what had been, from the start, a plot against Cromwell.

And then, all at once, I saw what had happened. What and how, though not whom. My heart began to beat excitedly. I turned the theory over in my mind half a dozen times. It fitted the facts better than anything else. Abruptly I got up and left the inn, so preoccupied I stumbled into the giant's bone on the way out, setting it swinging once more in its chains.

I WALKED RAPIDLY to Joseph's lodgings, to fetch Genesis from the stables. The horse was waiting in his stall, placid as ever. As I rode out I glanced back at the building; it was a poor enough place, but it would be costing Joseph far more than he could afford. Faithful, tenacious Joseph, how his enthusiastic godliness and fussiness irritated me sometimes. Yet he had been utterly steadfast in his loyalty to Elizabeth. I should have gone to the Wentworths' house today, but I realized I wanted Barak with me when I did. Guy was right: there was real evil in that house. And I saw that, if my theory was correct, we could still rescue Cromwell from his plight. There was no need for more secrets.

Barak was not at home when I returned. I waited impatiently for two hours as the sun slowly set. I remembered my warning to him earlier, and hoped he had not met with danger. It was a great relief when at last I heard him come in and throw off his boots. I called him into the parlour.

'Not more bad news?' he asked, looking at my flushed face.

'No.' I closed the door. 'Barak,' I said excitedly, 'I think I have worked out what happened. This afternoon I went back to that tavern, the one where we met the sailors. There was a giant's bone hanging from the ceiling, do you remember that?'

He raised a hand. 'Wait. You're going too fast for me. What's the giant's bone to do with anything?'

'It was something the landlord said. "Better to have had the whole skeleton, but that one bone's enough to bring people here to look and that's good for trade." That set me thinking – my mind has been too full for proper thought, that's why I didn't make the connection between the Bealknap case and Richard Rich. Listen, we've wondered all this while why the Gristwoods waited six months between finding Greek Fire and going to Cromwell. Especially when according to Bathsheba they were plotting against him from the start.'

'Ay.'

'The Gristwoods knew, when they first stumbled on Greek Fire at Barry's, that this was something very big. And very profitable. Michael Gristwood worked at Augmentations and he would have known the anti-Cromwell faction was growing.'

'Everyone knew that.'

'So I think they decided to offer it to someone within the anti-reformist faction as something they could take to the king and use to advance themselves. Again, everyone knows the king's interest in ships and weaponry. The Gristwoods probably thought it was safer to be in with the coming faction.'

'Then who?' Barak asked, excited himself now. 'Marchamount? He's a protégé of Norfolk's, the earl's biggest enemy.'

'Possibly. Though, being at Augmentations, Michael had a channel to Rich and Cromwell says Rich is plotting. This puts him and Bealknap back on the list.'

'Then we have to include Lady Honor too. She's no reformist.'

'All right, for the sake of argument. At all events, the Gristwoods went to someone. Call them Cromwell's enemy for now. They took the barrel and the formula, and promised to make more Greek Fire for them. Toky and Wright were set to work to help them and probably to keep an eye on them too.'

'Yes, that fits.'

'So for six months they try to make more Greek Fire. But the stuff is like nothing they've ever seen and the formula, perhaps, referred to the use of an element they didn't have. I wondered earlier why the Romans, who knew of something like Greek Fire, didn't develop it as a weapon. There were sources, pools of strange flammable liquid in the ground, which the Byzantines had access to but the Romans didn't. Far beyond Jerusalem. And we don't have access, either, to whatever it was.'

His eyes were wide with interest now. 'Something essential to make Greek Fire?'

I nodded. 'I see Michael and Sepultus following all sorts of trails, like the Polish drink, trying different experiments, increasingly desperate.'

'Because they couldn't make Greek Fire despite having the formula.'

'Exactly. And how frustrating that must have been for them, and their masters, to have this opportunity for such power and wealth just beyond their grasp. Remember that they had reconstructed the apparatus that was used to project Greek Fire with the aid of Leighton the founder, and practised in his yard using the stuff in the barrel. They knew it worked. How frustrated, and how angry, they must have become as the winter passed and Cromwell found himself in ever greater trouble over the Cleves marriage.'