"Morning, Doctor," Sarah Jane Smith said as she walked into theTARDIS control room, rubbing the last traces of sleep from her eyes. "How are things?"

"Hmm?" the Doctor responded, not looking up from the central console. "Oh, good morning, Sarah. You're just in time for us to land."

"I wonder what trouble we'll find ourselves stuck in this time," Sarah remarked. "I mean, look at some of the things we've run into lately - anti-matter monsters, ancient Egyptian gods...what's next?"

The Doctor was wearing his mischievious, slightly manic grin as he looked up at her, his eyes wide and twinkling. "Why don't we take a look at just what awaits us, eh?" As the column in the center of the console ceased its up-and-down motion, the Doctor flicked a switch, activating the main viewscreen - which revealed only the blackness of interstellar space, flecked with stars and distant, swirling nebulae.

Despite herself, Sarah was a tiny bit disappointed. It wasn't that she didn't find such sights pleasant, but she had been expecting something more exciting. "Where are we...and when?" she asked.

"Hovering in deep space, sector 311-C4," replied the Doctor, peering at the instruments. "Several million years in your past, it looks like."

Sarah stepped closer. "Anything interesting happening around this time?"

The Doctor shrugged. "Well, on Earth, it's probably the height of the Silurians - also known as Earth Reptiles, or homo reptilia. Fascinating people, capable of resurrecting entire extinct species...until certain events forced them underground. Elsewhere in the universe, the Carrionite-Hervoken war is beginning, maybe. Can't quite remember about the Sontaran-Rutan war..."

"Doctor, there's something out there!" Sarah suddenly exclaimed, pointing at the screen. Against the starfield, an object was now coming slowly into view, spherical and grey-white.

The Doctor was checking the scanners. "From what I can tell," he said, "it appears to be a small planetoid or moon...wandering through space by itself."

Sarah looked at him, then back at the object in amazement. "It's just drifting, not orbiting anything? But that isn't possible!"

"Oh, you'd be surprised, Sarah," the Doctor told her. "It isn't unheard of for whole planets to break their orbit and just go coasting through the interstellar void, for one reason or another." He looked up from the scanners then. "Now this is interesting...our wandere has the exact same dimensions and composition as Earth's moon. Come on, Sarah, we've got some investigating to do."

OOOOOOOO

A few moments after the TARDIS materialised on the wandering moon's rocky surface, the Doctor and Sarah stepped out, both wearing atmospheric suits. "Well, it certainly looks like Earth's moon," Sarah said, her voice slightly distorted by the microphone in her helmet, as she took her first awkward steps across the landscape.

"Yes," replied the Doctor, "but if it is, what sent it out here in the first place? It was born from the Earth, orbited it long before this time - I should know, I was there - but then it was its' arrival from space that ended the first Silurian civili - " He stopped abruptly then, and swayed slightly.

"Doctor?" said Sarah. "Doctor, are you alright?"

"Sarah..." The Doctor's voice was barely above a whisper. "There's a presense here...a consciousness, an intelligence...our thoughts are touching. It's speaking inside my mind."

Sarah felt a twinge of worry. Was this thing, whatever it was, trying to take over the Doctor? Was it hostile? "Doctor, what does it want? Is there anything - "

"Shh!" the Doctor hissed then. "I'm having a conversation..."

OOOOOOOO

"There's no need to be concerned for me," the Doctor told Sarah back in the TARDIS, removing his helmet and placing it on a chair. "I'm perfectly alright." Taking her helmet off, Sarah looked closely at him; his eyes carried a hint of sadness.

"What was it?" she asked him.

The Doctor was not looking at her as he spoke, peering into his own unfathomably deep thoughts. "A being from another dimension, Sarah. An explorer. It found itself trapped on this plain of existence, weak and bodiless, already dying. To survive, it had to bond to something, and so it chose an object in the immediate vicinity at the time."

"The moon!" Sarah realised. "So, after it had merged with it, this alien took it out of Earth's orbit and into space!"

"It wanted to see the universe," the Doctor explained. "It's traveled a long way, and seen a lot...but the time it borrowed by bonding with the moon is ending, and it's dying. It hasn't long now. That's why it mind-linked with me, to tell someone of itself and its' experiences before its' inevitable demise."

Hearing this, Sarah felt some of the sadness the Doctor was experiencing. "Sort of like you, in a way," she commented. "It was curious about the universe around it, enough to want to explore."

"Yes..." the Doctor murmured, then started operating the controls. "Time for us to be on our way, I think."

OOOOOOOO

The alien consciousness observed the time travelers' craft disappear from the surface of the body which had sustained it for so long, glad for having had the opportunity to communicate with the Doctor. Now it found it was not quite so bitter and fearful about its' impending death, but rejoiced in the certainty that it would be remembered. It felt its' life dimming, but was at peace.

When the consciousness finally died, the moon was left to drift, carried by cosmic winds. Eventually its path took it into a star system, and in the direction of the third planet from the sun...and its destiny.