An idea that's been rolling round in my head for a while now - all to do with timelines and parallel Doctors and exactly where the parallel universe became parallel. Tis all very confusing to put down in words. If anyone feels like asking, you're very welcome to PM me about it.

Rose could see it. Every time the Doctor looked up at the night sky she saw the longing in his eyes. He didn't do domestics – not because he couldn't, but because it was too. . .stationary. Much better to be travelling the whole of time and space, saving people, thwarting evil schemes, battling monsters, and running, always running. Here, stuck on a parallel Earth without a way to reach those stars the Doctor was so fond of looking at, Rose could see the man she loved wilting. At least Pete's mansion was in the middle of the countryside so the sky was clear.

'Doctor?' Rose asked one autumn night, about a year since they had been left at Bad Wolf Bay. 'Are you all right?'

The Doctor turned, flashing her that boyish grin that she loved so much. Just like a kid on Christmas. 'Yeah, 'course. Fine. Never been better. Why wouldn't I be, when I've got you?' he replied at ninety miles an hour. He took her hand, the same one that he had been created from, and kissed it, his chocolate eyes never leaving her face. He grinned again as a delicate blush stole over Rose's cheeks.

'No, I mean, are you all right with. . .living here. On Earth, in one place,' she clarified shyly. The grin disappeared, before returning to light the Doctor's face with false chipper.

'Remember that time when we were stuck on that planet with the Ood?' he asked suddenly. Rose grinned and nodded, noticing the change of subject but happy to get the Doctor back to memories. He was always happier then. 'And we thought we'd have to settle down?'

'Oh yeah! And I told you you'd have to get a mortgage!' Rose liked that memory – aside from the whole Satan-killer-Ood-almost being-sucked-into-a-black-hole part.

'Yeah, that's right,' the Doctor said fondly, swinging their arms between them. They fell silent for a moment as both were lost in the swirl of their former lives. The black swathe of the star dusted universe winked above them. A shooting star flashed in the north, the remains of some space rock burning up in the upper atmosphere from the incredible pressure of its velocity compressing the air in front of it. The Doctor didn't say this part out loud though, knowing it would spoil the moment for Rose, who had just cried out 'Quick! Make a wish!' tugging on his arm like she was ten. He turned, smiling at her gently. 'I have everything I could ever wish for,' he assured her, but it couldn't stop one tiny part of his vast and brilliant mind from wishing for a way to travel time and space again.

It had been bothering Rose for a while. A small niggling idea that refused to go away and yet refused to become a proper revolutionary brainwave. The Doctor now had a job as a scientist at Torchwood London, and she, as the highest authority the government could find on alien life and paranormal occurrences, had long ago taking the top job. The Doctor worked for Rose Tyler. He didn't seem to mind. He was careful not to discover any new technology that could advance the parallel human race too far, but of course he had to replace the sonic screwdriver that the original Doctor had taken with him.

On impulse, Rose checked UNIT's database. As head of Torchwood she had access to all personnel files, operations and information for all government, but it was rare that she used them. The job of monitoring timelines meant there was little use for such things.

'The Doctor'. There was a hit. So she had been right. There was a Doctor in this universe. Torchwood had been created from actions caused by the Doctor (and Rose herself) back in the 1800s with Queen Victoria and the werewolf – or the lupine wavelength haemoveriform as the Doctor had called it – when the two parallel universes had been the same. The split had happened after then. Using her superb resources, Rose was able to track down exactly when the two parallels had been created: the day she had been born.

The Doctor had once said that the world just reconstructed around the minute difference – in one, Rose lost her father in a traffic accident – in the other, she had never been born and her parents had become rich. But there was still a Doctor.

Maybe if she traced the adventures she had had with the strange occurrences happening in this world, she could find him. Or, at least, traces he left behind. There were animated plastic mannequins, Reapers, and even the Slitheen spaceship crashing into Big Ben. The Sycorax had come, but Torchwood had destroyed their ship. There had been no Doctor to stop them.

'Oh my God,' Rose whispered as the last piece of information clicked in her head. The half formed notion had suddenly become whole.

The Doctor had been in this parallel universe, probably the last survivor of the Time War as he was in the other world. He had come to Earth and fought the Autons, perhaps picking up another companion in the absence of Rose. They had gone to Satellite Five, seen the Dalek army in the far distant future. The Doctor must have sent the companion home on the TARDIS, but she hadn't managed to tap into the Time Vortex and save the Doctor's life. So the Doctor was now dead, and the TARDIS. . . Rose remembered what the Doctor had told her: Just let the TARDIS die. Just let this old box gather dust. . .Let it become a strange little thing standing on a street corner.

'Oh my God,' she whispered again. The TARDIS, or rather, this world's TARDIS, was here, in London. 'Doctor!' she called into the intercom.

'What is it?' the Doctor replied immediately, hearing the uncertain tremor in her voice.

'I need to tell you something.'

'I'm right there.'

Rose was pacing by the time the Doctor reached her office. She looked up as he entered, the old spark of adventure lighting her eyes. She looked beautiful. Quickly she relayed her revelation to him, doing an astounding impersonation of a Timelord hyperactive on jelly babies. It didn't even take a second for the implications of this discovery to snap into place in the Doctor's brain.

'Fantastic!' he beamed, picking Rose up and twirling her round. 'Rose Tyler, you are absolutely, one hundred per cent fantastic! Or should that be brilliant? What about brilliantly fantastic? Fantastically brilliant?'

'I like that one,' Rose grinned. It was just like old times. And the Doctor was grinning again, talking too fast for comprehension.

'Come on,' he urged, grabbing her hand.

'What, now?' she checked. There were reports to file, sanctions to authorise, new staff to interview.

'Of course now! When else?' the Doctor wouldn't be cowed. So they ran all the way to his lab, a huge room filled to the brim with odd bits and bobs and trinkets of alien technology. His workbench was cluttered with junk and microscopes. 'Rose, give me your key,' he instructed.

'What?' she asked, confused.

'Your TARDIS key, you still have it?' She nodded. 'I need it to locate the TARDIS.'

'Okay.' She took the key off her bunch, grinning even broader as she watched him take out those thick rimmed glasses and set to the key with the sonic. She could never tell the Doctor that for a long time she had stared at that key when there was nothing else to do, waiting and hoping to hear the whir of engines and see the key glow, signalling the return of the first Doctor, come to take her away from the parallel world.

'You see,' the Doctor garbled, interrupting her thoughts, 'Since the TARDIS in this world is molecularly equivalent to the TARDIS from the other world, and this key is a part of that TARDIS, I can use a beta wave distortion to alter the lines of temporal flux around the molecules of the key, turning it into a sort of homing beacon which will increase in resonance the closer it gets to the TARDIS.' He paused, since had said all of that really quickly in one breath and needed air. 'And so,' he concluded. 'If I just do this –' he fiddled with the setting on the screwdriver and blasted the key with it – 'there! The energy transformed from the shift in the lines of flux should now mean it glows brighter the closer we get. Now, where do you propose we start?'

It didn't take them long to reach the Powell Estate. The Doctor had turned his key into a TARDIS detector as well, to give them a better chance of finding it.

'Okay, so what exactly are we looking for?' Rose asked.

The Doctor shrugged. 'Could be anything. We don't know if this Doctor fixed the chameleon circuit or not. Just look out for anything.'

So they split up.

Rose started at what would have been her flat and worked backwards, looking out over the Powell estate to look for anything unusual. However, she knew it wouldn't be that easy: the Doctor had once told her about how the chameleon circuit worked. Long story short, the TARDIS had a kind of perception filter on it, so it wouldn't be noticed until up close if you were looking for it.

The Doctor meanwhile was running backwards and forwards in no particular way, watching his key intently. Strangely, he never banged into anything. A policeman standing at the gate of the park watched this display of random behaviour for several minutes before deciding it might put the public at some risk, and thought to intervene.

'Can I help you sir?' he asked politely.

The Doctor looked up, blinking through his thick rimmed glasses, surprised at being addressed. 'Hmm? Sorry, I wasn't listening,' he said cheerfully.

'I said "can I help you"?' the policeman repeated.

'Um,' replied the Doctor, scratching his head. 'Now you mention it, yes – yes you can. Have you seen a big blue box, wooden, says "POLICE" across the top?'

'You mean the sculpture?'

'Sculpture?' the Doctor asked, confused.

'Crikey, where've you been?' the policeman cried. 'There's been a blue box standing next to the playground on Par Street for nearly three years. Just appeared one day. People reckoned it must be modern art, part of the clear-up-the-city campaign or something. Oi! Where are you going?' But the Doctor was already too far away to hear him.

As he sprinted, the Doctor removed his newly acquired mobile phone (incidentally the only technology he wasn't able to work) and dialled Rose's number.

'Hey Doctor,' she greeted.

'Hey, Rose, I know where the TARDIS is – it's on Par Street, across from the playground. I'm about thirty seconds away.'

'Really? You're just round the corner from me then.'

'Why, where are –' But at that moment the Doctor rounded the corner and saw his blue police box with Rose leaning nonchalantly against the wooden door, grinning insanely. Sure there was graffiti all up the sides and somebody had kicked in the light on the top, but apart from that, it was just the same as the TARDIS of the other universe. He didn't realise until that moment how much he had missed it.

Rose saw his joyous expression and laughed quietly. 'I thought I'd wait for you to look inside,' she explained. The Doctor was now running his hands over the doors, crooning to it. The key on its chain glowed brilliantly and Rose could have sworn she heard an answering thrum from the long dormant engines of the TARDIS. She had never seen him this exuberant before.

He turned to her with such a huge beaming smile that it was impossible to tell whether his face had permanently split in two or not. 'Shall we go in?' he suggested politely.

'I think so,' Rose replied cordially. She was so pleased that the glimmer of adventure had finally returned to the Doctor's eyes.

He slotted the key into the lock – a perfect fit – and turned the lock. The door of the parallel TARDIS swung open with a wooden creak. All was darkness within.

'Ladies first,' said the Doctor graciously.

'Age before beauty,' Rose shot back with a smirk.

'Oi! Are you saying I'm not pretty?' The Doctor's expression was mortified.

'I think you're extremely pretty,' she replied with a swift kiss. 'But you are at least nine hundred and seventeen years older than me.'

'I think you'll find, Miss Tyler, that I am only three years old.' Rose opened her mouth with a retort, but closed it again. He had her there. So she folded her arms and pouted, making the Doctor laugh out loud. 'All right, we'll go in together.' He took hold of her hand and squeezed it. 'Allons-y!'

The inside of the TARDIS lit up as it detected intruders. To Rose the interior looked exactly the same – blue lights, DNA shaped staircase, central console and all – but to the Doctor there was a strange vibe that he didn't like at all. It was cold, lonely. This TARDIS missed its Doctor. Lovingly he touched the beams that supported the main chamber. The engines hummed slightly in recognition.

'So, where do you want to go first?' he asked suddenly, tearing himself away from the beam.

Rose considered for a bit. 'We never did go to Barcelona did we?'

More happy endings! And sometimes a little technobabble is good for the soul!