In the chair behind his cluttered desk, Merlin sat in an apparently relaxed position, his hands clasped together on the parchment pages of the book open before him. He was gazing across the room at her, impassive.

He looked tired, the sharpened planes of his face telling of too many long hours of study without food or sleep. His deep-set black eyes burned with the inner energy that was always a part of him, but there was something else, something even more vibrant than she was accustomed to seeing radiating from him.

"It's been more than twenty-four hours," she offered.

He was mildly surprised. "Oh? I hadn't realized."

"You have to eat."

"Do I?"

She blinked. "Don't you?"

With a sudden, slightly rueful smile, Merlin said, "Of course I do. But I'm not hungry right now. Sit down, Serena. We have to talk."

Those four little words were enough to make her feel extremely apprehensive, and his smile didn't reassure her a bit, but she removed several books from the chair nearest his desk and sat down.

Muster and Apprentice, that's all we are. Master and Apprentice. There can't be anything else.

With forced lightness she asked, "How was the Council meeting?"

"Difficult." Without elaborating, Merlin abruptly changed the subject. "Do you trust me?" he asked her.

"Of course." Her answer was instant, unthinking, and she felt an odd jolt when he seemed to wince. Was her answer unexpected, or simply unwelcome? She didn't know. His features smoothed out quickly, and his voice was calm when he went on.

"Good. Because I'm going to have to ask you to hold on to that trust with both hands."

She eyed him warily. "Why? Are you going to do something to make me doubt I can trust you?"

"I hope not," he murmured, then shook his head a bit. "Serena, I can't explain everything just yet. I know you're tired of hearing that, but please try to be patient. I have my reasons, and they're good ones. You have to trust me on that point."

"All right," she said, slowly and reluctantly-but he definitely had her interest. Since she did trust him, it didn't seem too much to ask. For the moment.

"Thank you. If it helps, I believe you won't have too many questions left by the time we get back."

"Back? Where are we going?"

He looked down at the book lying open on his desk. "We're going through a gate, Serena. A gate into time."

That surprised her so much that she could only stare at him when he went on somewhat broodingly.

"We'll have to be careful. Our presence alone could have unimaginable consequences. To change the past is to change the present. And the future."

Serena was trying to fathom the undercurrents she could barely sense in him. It was as if he had severed some tie, burned his bridges behind him, and that unnerved her. How much was she to take on faith? Everything? Or could she ask questions? Uncertain on that point, she opted for a simple statement. "You haven't taught me about time travel."

"Of all our abilities, it's the most dangerous." His gaze turned to her, still brooding. "It's also forbidden without the approval of the Council."

"Do we… have its approval?"

Merlin shook his head.

All Serena knew of the Council of Elders was that it was the ultimate authority among wizards. Merlin hadn't told her much more than that. But it was enough to make her feel a little chilled just then.

She attempted a laugh that didn't quite come off. "You, Richard, breaking a rule?"

His mouth twisted oddly. "If I had not broken another… Well, never mind that now. I'll deal with the Council, if need be, when we return."

"This is very important," she realized.

"Very."

"Why?" She wasn't sure he would answer.

Merlin hesitated. "The less you know about the specifics, the better our chances of success."

"Really?" She couldn't help doubting that.

"Really, Serena. I'll take an oath if you like, but I hope you won't need that from me. The truth is I honestly believe that for you to know everything at this point is to invite potential disaster."

The sincerity in his voice convinced Serena he meant what he said. It was frustrating, but she had to trust that he knew what he was doing. "All right. What can you tell me?"

Merlin obviously chose his words carefully. "I believe that something went wrong in our past."

"Our past?"

"The past of wizards. I can't be sure, since it was so long ago and most of the records haven't survived- either because of the passage of time or because they were deliberately destroyed. All I am certain of is that we must go back and try to understand what happened."

She frowned. "And change it?"

Again he hesitated. "I don't know. That decision can only be made when we have more information. If we make a mistake-change too much or the wrong thing-we could destroy our present."

Serena felt another chill. "If we did that-made a mistake in the past, I mean-then couldn't we go back again and just fix the mistake?"

Merlin shook his head. "Not even a wizard can exist twice in the same time and place. Paradox: the bane of time travel. Once we go back, then we were there."

"Yes, but…" Serena chewed on her bottom lip as she tried to figure that out.

Patiently Merlin said, "There are two paradoxes in time travel. The first is our inability to alter our individual lives-our personal time lines-in any way whatsoever. Any change, however minor, affects who and what we became; that, in turn, affects our reason for going back in the first place."

Serena blinked. "Um… I'm confused."

He smiled briefly. "All right, then consider the example today's thinkers like to offer when they say time travel is impossible. Suppose you build a time machine, and it takes you back along your personal time line- which is, in effect, the direct line of your ancestry. You encounter your father years before your own conception. Either directly or merely by your presence, you influence events in his life, and he dies."

She waited, then said, "And so?"

"And so it isn't possible. If your father dies before your conception, then you are never born to build a time machine and travel back in time. Paradox."

That example worked. Serena nodded slowly. "I get it. We can't do anything that would directly affect our own present, because it would change too much for us to be able to go back."

"Close enough," Merlin murmured.

"But you said there were two kinds of paradoxes. What's the second one?"

"In a sense the second is much simpler. Once we go back, we were there. What do you suppose would happen, Serena, if you went back to the same place a second time and came face-to-face with yourself?"

She shivered. "That's eerie."

"It's also dangerous. The theory is that a duplication of self occupying the same place and time would fracture that time line. Destroy it-or unalterably change what must be."

Serena cleared her throat. "So what would happen to me in that case? Both of me?"

"I can only offer you another theoretical answer. In theory, there would be, from that point on, two separate Serena's in two separate-and probably quite different-time lines. Alternate lives, alternate futures, and both of you would be diminished."

"Yuk." She stared at him. "I don't like the sound of that at all."

"I should hope not."

"So we only go back once."

"We could go back to an earlier or later time, or another place in the same time, but we aren't allowed the luxury of repeating our actions until we're satisfied with them." He looked at her steadily. "It's a one-shot deal, Serena. We have to get it right the first time."