David stared. Was this scruffy old man really telling him that he’d had a dream about the past and somehow actually gone there? Become, in fact, part of history? Being kidnapped was bad enough, but being held by a madman was far worse. David jumped to his feet and yelled at the professor.

“You’re crazy! You have no right to lock people up and tell them stupid stories. Why should I believe anything you tell me? I just had a bad dream, that’s all. It was just a dream!”

“There is no such thing as ‘just a dream’ for people like you and me, David,” said a female voice, and David remembered that there was someone else in the room. “What the professor has told you is true,” she went on, with what sounded like a German accent. A girl stepped forward out of the shadows.

She was about the same age as David, or perhaps slightly older, and was wearing some kind of black jumpsuit. Above her green eyes and pretty smile, she had a mass of loose brown curls. David was instantly sure he’d seen her somewhere before, and he sat back heavily into his chair when he got it. It was the girl who had called to him in the passageway outside his school. The girl who’d seemed to vanish into thin air. She smiled at him again and held out her hand.

“My name is Petra. I’m a dreamwalker, like you.”

“A dreamwalker?” David took the slim hand and somehow remembered to shake it. “But what … ?”

“What is a dreamwalker?” Petra’s eyes danced with amusement. “Well, you know what a sleepwalker is, don’t you? Someone who walks while they are sleeping? Well, I do the same thing while I dream. And so do you.”

David stared at the girl, then at the old man, then back at the girl again. They were obviously quite serious about what they were saying.

“But … what does that actually mean?”

“It’s not as complicated as it might sound,” said the professor, in a teacher’s manner that clearly came naturally to him. “What do you think a dream is, David?”

“Er … I suppose I always thought dreams were just a jumble of stuff from the day. You know, memories and sights and sounds, all muddled together.”

“Well, that’ll do for a start,” said the professor. “On the surface a dream is just that — a mixed-up collection of recent experiences. But doesn’t that sound rather boring? I mean, have you never wondered why our dreams are so much more interesting and odd than our actual daily lives? What about those things that don’t come from your recent experience — the strange people and places that can suddenly pop into your dreams from out of nowhere? Have you never wondered about them?”

“No,” said David, “not until now, anyway. Although … yes, I did sometimes wonder about my Eddie dreams. They always seemed so real.”

“Ah!” said the professor. “Good.” He removed his glasses and began cleaning them on his tie. “You see, there’s more to what we see in our sleep than just recent experiences being played back in our heads. In a sense a dream is also a window for the mind, allowing us fragmentary glimpses of the waking world beyond our skulls. At least, that’s the case for most people. For you, though, the effect is stronger — much, much stronger. For you and Petra and others with your gift, that window is more like a doorway, a doorway that allows your mind to leave your sleeping body altogether. We call this dreamwalking.”

“Oka-a-a-y.” David folded his arms and scanned their faces for some sign that they were making a fool of him.

“All right,” said the professor with a chuckle, “we can go into the theory later. In practical terms, what it all comes down to is this: If I dream about Stonehenge or the Eiffel Tower or Mount Rushmore, it’s all just in my head, okay? But if you dream about these places …”

David glanced over at Petra. She was watching him as closely as the professor was.

“You’re saying I can actually go there?” David said. “But that’s crazy.”

“It may sound crazy to you now, but that’s exactly what I’m saying,” said the professor. “At least, your mind can actually go there. Your body stays exactly where you left it, fast asleep. Now, earlier Roman asked if you’d ever seen a ghost. So … have you ever wondered what ghosts are?”

“Not really. I just thought they were meant to be the spirits of dead people.”

“I don’t know anything about the dead,” said the professor, “but I can tell you that some ghosts at least belong to the living. You see, that’s how you appear when you dreamwalk. Like a ghost.”

“But …” David shut his eyes for a moment as he tried to make sense of what he was hearing. “… my Eddie dream — you said I was back in the year 1940.”

“Yes!” The professor’s face lit up, like a kid who’d just been reminded it was his birthday. “And that’s the real wonder of dreamwalking. Don’t you see? It’s not just that you can go anywhere you want, it’s also anywhen. Freed from the body, your dreamwalking mind can ignore the laws of nature, David, even time itself.”

“You’re … time-travelers?” said David, amazed as the thought finally hit him.

“No,” said the professor, the look on his face becoming suddenly wistful. “You are. I can only dream.”

Petra shifted uncomfortably in her seat. “All this talk! Professor, wouldn’t it be easier to just show him?”

“I’ve already made arrangements for that,” the professor replied. “David, you’re going to need to accept all this as quickly as possible if you’re going to be any use to us. So, what do you think? You’ve been dreamwalking accidentally for about a year now, but are you ready to do it deliberately? Are you ready to free your mind, to become — in a sense — your own ghost?”

David didn’t know what to say, but if these two were playing some trick on him, they were very good actors. And suppose it was true? What the professor had said about Eddie certainly sounded right. The dream-Eddie had always seemed the scientist type — constantly asking questions and never just accepting the answers, writing everything down in those notebooks of his. His own grandfather! Besides, no matter what was coming next, no matter how incredible it all sounded, there was no way he was going to look bad by showing fear. And he couldn’t help liking the way Petra was looking at him.

He nodded. “You said I can be useful in some way. How?”

“Come with me,” said the professor, standing, “and I’ll show you.”