Disclaimer: Sadly, I have no claim on Doctor Who and make no profit from it.
If my explanation of how Time Lords can travel interdimensionally contradicts any canon that I am unaware of, well, let's just call it AU.
One week later, Rose was standing in front of the TARDIS, duffel bag on her shoulder. The Doctor had said that he needed some time to make the arrangements. And truth be told, despite her eagerness to see her Doctor again, she had needed some time as well. There would be no coming back from this adventure, and she owed it to everyone to tie up loose ends before she left.
She had asked her family and Mickey to come with her, but had not been at all surprised when they turned down the offer. This was Pete Tyler's world, after all, and that made it Jackie's world as well, especially now with the baby binding them together. And Mickey had his gran and his work at Torchwood, giving him a sense of direction and purpose that he had never found back in the Powell Estates.
What did surprise her was how easily they had accepted her decision. Oh, there had been plenty of tears and plenty of hugs, but very little attempt to change her mind. They would miss her terribly, but everyone knew where she belonged. She had built an amazing life in this universe, one that would make the Doctor beam with pride, but they all understood that her rightful place was at his side. "Just think of it," Mickey had told her with a bittersweet smile at their final farewell. "The Doctor and Rose Tyler, in the TARDIS. All is right with the universes once again."
And so with a deep breath, she raised her hand to knock on the door, but it flew open before she had the chance. "Ah, you're here, fantastic! Welcome! Come on in! Rose Tyler, meet my friend, Adam Mitchell."
She stepped through the door and then froze momentarily as she was confronted with a face she had never expected to see again. Without a word, but with a look to rival that of the Oncoming Storm, she marched up the ramp, ignoring Adam's extended hand, and snapped her fingers in front of his eyes. He blinked in surprise, and then turned to the Doctor. "Eh, you didn't mention that she's a bit off her rocker."
"Well, you don't have a door in your head, that's something," she said, her hostile expression thawing by only a degree or two.
"No, I don't suppose I do. Why, do you? Is that like a thing back in your world?" he asked, confused.
She ignored the question and turned to the Doctor. "He can't come with us. We had him with us for a bit, and he nearly got us all killed in this giant broadcasting station because he decided to get a port installed in his head so that he could download future technology secrets and resell them."
The Doctor watched her calmly, compassionately. "Remember, Rose: different universe, different person, different choices."
"Wait," said Adam behind her with an incredulous laugh. "Do you mean Satellite Five? What was that, the Fifth Great and Beauteous Human Empire?" ("Fourth Great and Bountiful," corrected the Doctor automatically.) "Yeah, that was my first trip with the Doctor. I remember those people with the holes in their foreheads. Freaky things, those. Don't think I wasn't tempted for a just a moment, all that science and technology just waiting to be downloaded, but to have that kind of knowledge 200,000 years too early…well, who knows how it could tear apart the whole course of human history."
Rose turned back to him, surprised. "You got that? You really got it? The other Adam had no idea what he had almost done, even after the Doctor explained it to him. Actually tried to blame the Doctor for not keeping a better eye on him."
"Adam's been with me for a year now, Rose," the Doctor said softly. "He's a fantastic companion. I trust him with my universe; you can trust him with yours."
She studied the Companion in question for another moment, and finally her face softened and she held out her hand. "Right. Sorry, mate. It was just a shock to see that face again."
"No hard feelings."
"Right, then." She looked to the Doctor. "So how does this whole interdimensional traveling work exactly?"
The tension broken, the Doctor grinned and bounced over to the console. "All right, well, picture the fabric of the universe as some piece of knitting. Like a scarf or a cap. You try to just shove something through it, and you're going to tear a hole, or at the very least warp it out of shape, right? But if you are very careful and you tug just right on the yarn, you can create a space and slip something through, and then you massage and tug in another direction and you can close the space back up, yeah?" Rose nodded her comprehension. "Right, so there are four TARDISes out there right now and they are tugging in four directions to create a little slot for us to slide through. We pop across to your universe, deposit you with your Doctor, and then slide back through the same spot in space and time. The other Time Lords massage their threads of the fabric back to the proper place, and the hole closes up as if it were never there."
"And this TARDIS will still have power? We lost power when we came here because she couldn't use the energy of this universe."
"As long as my people are holding the hole open, she'll be able to continue drawing energy from home. The tricky part is the navigation. I've got to hit the exact right point in space and time twice – on the way there and on the way back."
Thinking of her Doctor's sometimes questionable navigation skills, Rose's smile faltered a bit, but she managed brightly, "If anyone can do it, it's you, Doctor." He beamed under her praise, looking so like her Doctor that her heart clenched. Soon. You'll see him soon. "So how did you convince four Time Lords to help some human girl they'd never heard of?"
"Four? Try twenty-four. Each TARDIS has six pilots. I told the Council that you were a space-time anomaly that had the potential to cause the collapse of the universe if you weren't returned to your proper place."
"And they bought that?" she asked incredulously.
"Oi, I don't know that it was that far off from the truth! I saw the look in your eyes. If we didn't take you back, I think you would have found a way to punch right through the Void all by yourself," he told her with a grin. "Although truth be told, they probably mostly agreed so that they could show off their space-time-manipulating skills. But no matter, they're here now. Are you ready?"
She couldn't even speak, just gave a squeak of excitement and nodded.
"Well then, hold tight. It's going to be a bumpy ride."
He hadn't been kidding. Travel in the TARDIS was rarely smooth, but this trip was downright bone-jarring. Only a few minutes had passed before the timeship settled into the comfortable rhythm of temporal orbit in the Vortex, but Rose had thought her teeth were going to rattle out of her head by then. Adam was the first to speak, although his voice was a bit strained. "Right, then, glad that's behind us. So where are we?"
"In the Vortex of Rose's dimension." The Doctor sounded like his normal self, but the paleness of his face and the sheen of perspiration on his forehead testified to how stressful the piloting had been.
"Really? We're really here? You're sure it's the right universe?" she questioned eagerly.
"Doubting me, are you? Let me double-check…Yes, the TARDIS says that your energy signature is aligned with this universe. You're home, Rose."
She squealed and launched herself into his arms; he laughed and spun her around. "Uh, not to throw a damper on the party, but it's a big universe. We still have to track this Doctor down," Adam warned.
"Oh, nothing easier! All TARDISes are linked, so we just have to lock ours onto the signal from his." The Doctor let go of Rose and bounced back to the console. "Here we go, old girl, go find yourself a new friend and…oh."
"Oh? What oh? That didn't sound like a good oh." Rose's high spirits faded instantly.
"Uh, well, she's not picking up any other signals."
She felt like someone had just knocked the wind out of her. "Did something happen? It's been so long, and he's always skating on the edge of disaster. Something has happened to him, to the TARDIS, hasn't it?" Hysteria began rising within her.
The Doctor gripped her shoulders. "Calm down, Rose. No use jumping to conclusions. It's much more likely that the link simply doesn't work transdimensionally."
"What about contacting him telepathically?" Adam supplied helpfully.
"Brilliant! I'll need a bit of peace; it could take a few minutes to reach him depending on when and where he is." He hopped up onto the jump seat and closed his eyes, brow furrowed in concentration. Rose paced back and forth, staring at him, for what seemed like an eternity, before he finally looked up. The instant their eyes met, she could tell by the compassion in his gaze that he had had no success.
"He's dead, isn't he?" she whispered, tears blinding her.
"Rose Tyler!" he said sharply. "I didn't take you for a pessimist! Tell me, how did I know you were from another dimension?" When she didn't answer, he demanded, "Come on, you can't have forgotten already. How did I know?"
In a small voice, she replied, "My energy signature was wrong."
"Right. Incompatible. Well, my TARDIS and I are in the wrong universe right now. Hardly a surprise that we would be incompatible. Just because we can't sense him doesn't mean he's not here, not by half."
He was right. No one had ever called Rose Tyler a pessimist. Even at that brief speech, she could feel the hope surging within her again. She blinked back the tears and managed a small but genuine smile.
"Uh, again, sorry to be the killjoy, but the fact remains that this bloke could be literally anywhere in time and space. So how do we find him?" Adam pointed out. There was a brief pause while they all contemplated this question, then Adam offered, "You're his counterpart. Where would you be right now?"
"Oi, what has that got to do with anything? Your counterpart went and put a door in his head. I have no idea what whims could seize this bloke at any given moment."
"How did you find the Nestene Consciousness?" Rose blurted.
"What was that?" He swung around to her, confusion on his face.
"You told me that you just stumbled across the Nestene Consciousness. How did you do it? How did you realize it was there?"
He still looked puzzled as to where this was going. "Rose, you said he's already battled the Nestene Consciousness here. We can't take you back to that version of him, it would throw the timelines completely off."
"I know that," she said impatiently, "Just tell me how you found it."
"Well, I was just fiddling around with the TARDIS, and I picked up this transmission. Couldn't tell exactly what it was, but I knew it wasn't 21st century Earth. So I went investigating, traced the signal back."
"Wait," she laughed, "Are you telling me that you found it by scanning for alien tech?"
"Uh, I suppose that's one way of putting it…"
"Oh, I love that!" She flung her arms around his neck, and then let go quickly, deciding that she was spending entirely too much time hugging a man who was not her Doctor but looked just like him. "Well, he's just as much of a magnet for trouble as you are. And he spends a good deal of time hanging around Earth. So I say that's as good a place as any to start. We go to 21st century Earth; we scan for alien tech; if we find something, we check it out; and I'll bet you anything we run into my Doctor doing the same thing."
The two men considered this for a moment and then broke into matching smiles. "Fantastic!"