She tapped one map. "Just for orientation, the monastery is here. Castelgard is over here, across the river. And the fortress of La Roque is on the bluffs here, above the monastery. Questions so far?"

They shook their heads.

"All right. The situation in the area is a little unsettled. As you know, April of 1357 puts us roughly twenty years into the Hundred Years War. It's seven months after the English victory at Poitiers, where they took the king of France prisoner. The French king is now being held for ransom. And France, without a king, is in an uproar.

"Right now, Castelgard is in the hands of Sir Oliver de Vannes, a British knight born in France. Oliver has also taken over La Roque, where he is strengthening the castle's defenses. Sir Oliver's an unpleasant character, with a famously bad temper. They call him the `Butcher of Crecy,' for his excesses in that battle."

"So Oliver is in control of both towns?" Marek said.

"At the moment, yes. However, a company of renegade knights, led by a defrocked priest called Arnaut de Cervole-"

"The Archpriest," Marek said.

"Yes, exactly, the Archpriest - is moving into the area, and will undoubtedly attempt to take the castles from Oliver. We believe the Archpriest is still several days away. But fighting may break out at any time, so we will work quickly."

She moved to another map, with a larger scale. It showed the monastery buildings.

"We arrive approximately here, at the edge of the Foret de Sainte-Mere. From our arrival point, we should be able to look right down on the monastery. Since the Professor's message came from the monastery, we will go directly there first. As you know, the monastery takes its main meal of the day at ten o'clock in the morning, and the Professor is likely to be present at that time. With luck, we'll find him there and bring him back."

Marek said, "How do you know all this? I thought nobody's ever gone into the world."

"That's correct. No one has. But observers close to the machines have still brought back enough that we know the background at this particular time. Any other questions?"

They shook their heads no.

"All right. It is very important we recover the Professor while he is still at the monastery. If he moves to either Castelgard or La Roque, it will be much more difficult. We have a tight mission profile. I expect to be on the ground between two and three hours. We will stay together at all times. If any of us is separated from the others, use your earpieces to get together again. We will find the Professor, and come right back. Okay?"

"Got it."

"You'll have two escorts, myself and Victor Baretto, over there in the corner. Say hello, Vic."

The second escort was a surly man who looked like an ex-marine - a tough and able man. Baretto's period clothes were more peasantlike, loose-fitting, made of a fabric like burlap. He gave a nod and a slight wave. He seemed to be in a bad mood.

"Okay?" Gomez said. "Other questions."

Chris said, "Professor Johnston has been there three days?"

"That's right."

"Who do the locals think he is?"

"We don't know," Gomez said. "We don't know why he left the machine in the first place. He must have had a reason. But since he is in the world, the simplest thing for him would be to pose as a clerk or scholar from London, on a pilgrimage to Santiago de Compostela in Spain. Sainte-Mere is on the pilgrimage route, and it is not unusual for pilgrims to break their trip, to stay a day or a week, especially if they strike up a friendship with the Abbot, who is quite a character.

The Professor may have done that. Or he may not. We just don't know."

"But wait a minute," Chris Hughes said. "Won't his presence there change the local history? Won't he influence the outcome of events?"

"No. He won't."

"How do you know?"

"Because he can't."

"But what about the time paradoxes?"

"Time paradoxes?"

"That's right," Stern said. "You know, like going back in time and killing your grandfather, so that you can't be born and couldn't go back and kill your grandfather-"

"Oh, that." She shook her head impatiently. "There are no time paradoxes."

"What do you mean? Of course there are."

"No, there aren't," came a firm voice behind them. They turned; Doniger was there. "Time paradoxes do not occur."

"What do you mean?" Stern said. He was feeling put out that his question had been so roughly treated.

"The so-called time paradoxes," Doniger said, "do not really involve time. They involve ideas about history that are seductive but wrong. Seductive, because they flatter you into thinking you can have an impact on the course of events. And wrong, because of course, you can't."

"You can't have an impact on events?"

"No."

"Of course you can."

"No. You can't. It's easiest to see if you take a contemporary example. Say you go to a baseball game. The Yankees and the Mets - the Yankees are going to win, obviously. You want to change the outcome so that the Mets win. What can you do? You're just one person in a crowd. If you try to go to the dugout, you will be stopped. If you try to go onto the field, you will be hauled away. Most ordinary actions available to you will end in failure and will not alter the outcome of the game.

"Let's say you choose a more extreme action: you'll shoot the Yankee pitcher. But the minute you pull a gun, you are likely to be overpowered by nearby fans. Even if you get off a shot, you'll almost certainly miss.

And even if you succeed in hitting the pitcher, what is the result? Another pitcher will take his place. And the Yankees will win the game.

"Let's say you choose an even more extreme action. You will release a nerve gas and kill everyone in the stadium. Once again, you're unlikely to succeed, for all the reasons you're unlikely to get a shot off. But even if you do manage to kill everybody, you still have not changed the outcome of the game. You may argue that you have pushed history in another direction - and perhaps so - but you haven't enabled the Mets to win the game. In reality, there is nothing you can do to make the Mets win. You remain what you always were: a spectator.

"And this same principle applies to the great majority of historical circumstances. A single person can do little to alter events in any meaningful way. Of course, great masses of people can `change the course of history.' But one person? No."

"Maybe so," Stern said, "but I can kill my grandfather. And if he's dead then I couldn't be born, so I would not exist, and therefore I couldn't have shot him. And that's a paradox."

"Yes, it is - assuming you actually kill your grandfather. But that may prove difficult in practice. So many things go wrong in life. You may not meet up with him at the right time. You may be hit by a bus on your way. Or you may fall in love. You may be arrested by the police. You may kill him too late, after your parent has already been conceived. Or you may come face to face with him, and find you can't pull the trigger."

"But in theory…"

"When we are dealing with history, theories are worthless," Doniger said with a contemptuous wave. "A theory is only valuable if it has the ability to predict future outcomes. But history is the record of human action - and no theory can predict human action."

He rubbed his hands together.

"Now then. Shall we end all this speculation and be on our way?"

There were murmurs from the others.

Stern cleared his throat. "Actually," he said, "I don't think I'm going."

Marek had been expecting it. He'd watched Stern during the briefing, noticing the way he kept shifting in his chair, as if he couldn't get comfortable. Stern's anxiety had been steadily growing ever since the tour began.

Marek himself had no doubts about going. Since his youth, he had lived and breathed the medieval world, imagining himself in Warburg and Carcassonne, Avignon and Milan. He had joined the Welsh wars with Edward I. He had seen the burghers of Calais give up their city, and he had attended the Champagne Fairs. He had lived at the splendid courts of Eleanor of Aquitaine and the Duc de Berry. Marek was going to take this trip, no matter what. As for Stern-