Bobby saw blurred images of a city: small, cluttered, crowded, of white stone that glowed in the flat sunlight.

“Jerusalem,” David said now. “Fifteen July, 1099. Full of Jews and Muslims. The Crusaders, a military mission from Western Christendom, had laid siege to the city for a month. Now their attack is reaching its peak.”

Bobby watched bulky figures clambering over walls, soldiers rushing to meet them. But the defenders fell back, and the knights advanced, wielding their swords. Bobby saw, incredibly, a man beheaded with a single blow.

The Crusaders fought their way to the Temple area. There the defending Turks held out for a day. At last — wading in blood up to their ankles — the Crusaders broke through and quickly slew the surviving defenders.

The knights and their followers swarmed through the city, taking horses and mules, gold and silver. Lamps and candelabras were stripped from the Dome of the Rock. Corpses were butchered, for sometimes the Crusaders found coins in the bellies of the dead.

And, as the long day of pillage and butchery went on, Bobby saw Christians tear strips of flesh from their fallen foe, smoke and eat them.

All this in violent, colour-filled glimpses: the vermilion splash of bloody swords, the frightened cries of horses, the hard eyes of grimy, half-starved knights who sang psalms and hymns, eerily, even as they swung their great swords. But the fighting was oddly quiet: there were no guns here, no cannon, the only weapons wielded by human muscles.

David murmured, “This was an utter disaster for our civilization. It was an act of rape, and it caused a schism between East and West that has never truly healed. And it was all in the name of Christ.

“Bobby, thanks to the WormCam, I’ve been privileged to watch centuries of Christian terrorism, an orgy of cruelty and destruction that stretched from the Crusades to the sixteenth-century plundering of Mexico and beyond: all of it driven by the religion of the Popes — my religion — and the frenzy for money and property, the capitalism of which my own father is such a prominent champion.”

With their mail and bright crosses the Crusaders were like magnificent animals, rampaging in the sunlit dust. The barbarism was astonishing.

But still…

“David, we knew this. The Crusades were well chronicled. The historians have been able to pick out fact from propaganda, long before the WormCam.”

“Perhaps. But we’re human, Bobby. It is the cruel power of the WormCam to retrieve history from the dust of textbooks and make it live again, accessible to our poor human senses. And so we must experience it again, as the blood spilled centuries back flows once more.

“History is a river of blood, Bobby. That is what the WormCam forces us to see. History washes away lives like grains of sand, down to the sea of darkness — and every one of those lives is, was, as precious and vibrant as yours or mine. And none of it, not one drop of blood, can be changed.” He eyed Bobby. “You ready for more?”

“David.”

David, you aren’t the only one. All of us share the horror. You are sinking into self-indulgence, if you suppose that you alone are witnessing these scenes, feeling this way.

But he had no way to say this.

David brought up another image. Bobby longed to leave, to turn his head away. But he knew he must face this, if he was to help his brother.

Once again, life and blood fled across the ’Screen.

In the midst of this, his most difficult time, David kept his promise to Heather, and sought out Mary.

He had never regarded himself as particularly competent in affairs of the human heart. So, in his humility — and consumed by his own inner turmoil — he had spent a long time seeking a way to approach Heather’s difficult, anguished daughter. And the way he found, in the end, was technical: through a piece of software, in fact.

He came to her workstation in the Wormworks. It was late, and most of the other researchers had gone. She sat in a pool of light, coloured by the flickering glow of the workstation SoftScreen, surrounded by the greater, brooding darkness of this dusty place of engineering and electronics. When he arrived, she hastily cleared down the ’Screen. But he glimpsed a sunny day, a garden, children running with an adult, laughing, before the darkness returned. She glowered up at him sulkily; she wore a baggy, grubby T-shirt bearing a brazen message:

David admitted to himself he didn’t understand the significance, but he wasn’t about to ask her about it. She made it clear, by her silence and posture, that he wasn’t welcome here. But he wasn’t about to be put off so easily. He sat beside her.

“I’ve been hearing good things about the tracking software you’ve been developing.”

She looked at him sharply. “Who’s been telling you what I’ve been doing? My mother, I suppose.”

“No. Not your mother.”

“Then who…? I don’t suppose it matters. You think I’m paranoid, don’t you? Too defensive. Too prickly.”

He said evenly, “I haven’t made up my mind yet.”

She actually smiled at that. “At least that’s a fair answer. Anyway, how did you know about my software?”

“You’re a WormCam user,” he said. “One of the conditions of use of the Wormworks is that any innovation you make to the equipment is the intellectual property of OurWorld. It’s in the agreement I had to sign on behalf of your mother — and you.”

“Typical Hiram Patterson.”

“You mean, good business? It seems reasonable to me. We all know this technology has a long way to go.”

“You’re telling me. The whole user interface sucks, David.”

“- and who better to come up with ways of putting that right than the users themselves, the people who need to make it better now?”

“So you have spies? People watching the pastwatchers?”

“We have a layer of metasoftware which monitors user customization, assessing its functionality and quality. If we see a good idea we may pick up on it and develop it; best of all, of course, is to find something which is a bright idea and well developed.”

She showed a flicker of interest, even pride. “Like mine?”

“It has potential. You’re a smart person, Mary, with a bright future ahead of you. But — how would you put it? — you know diddly-squat about developing quality software.”

“It works, doesn’t it?”

“Most of the time. But I doubt that anybody but you could make an enhancement without rebuilding the whole thing from the ground up.” He sighed. “This isn’t the 1990s, Mary. Software development is a craft now.”

“I know, I know. We get all this at school… You think my idea works, though.”

“Why don’t you show me?”

She reached for the SoftScreen; he could see she was about to clear the settings, set up a fresh WormCam run.

Deliberately he put his hand over hers. “No. Show me what you were looking at when I sat down.”

She glared at him. “So that’s it. My mother did send you, didn’t she? And you’re not interested in my tracking software at all.”

“I believe in the truth, Mary.”

“Then start telling it.”

He picked off the points on his fingers. “Your mother’s concerned about you. It was my idea to come to you, not hers. I do think you ought to show me what you’re watching. Yes, it serves as a pretext to talk to you, but I am interested in your software innovation in its own right. Is there anything else?”

“If I refuse to go along with this, will you throw me out of the Wormworks?”

“I wouldn’t do that.”

“Compared to the equipment here, the stuff you can access via the net sucks.”

“I told you, I’m not threatening you with that.”

The moment stretched.

Subtly, she subsided in her seat, and he knew he had won the round.