A/N: Sorry if the transition from the script to original is a bit rough…I wrote my part and then went back and went by the original. So yeah…Enjoy!

He was regenerating.
She hadn't seen it before, so he expected her to worry, though he believed that if he was one man one minute and one man the next, she would understand-at least, as much as anybody did without further explanation. He had tried to give her a bit of a warning, the statement that Time Lords had a way of cheating death, that he would change. But she would definitely be confused. He would have to explain.
The change concluded—the quick part, anyway. Feeling a bit odd as he always did after regeneration, he looked at Rose, trying to make sure she was okay.
"Hello. Oka—hmm." His new mouth caught him off guard. He felt his teeth with his tongue, trying to get used to them before speaking again. "New teeth. That's weird. So where was I?" Even his mind had to get used to the change for a few seconds before he remembered, grinning at his companion, who, he failed to notice, was still frozen, staring at him as though he were an alien—which, technically, he was—but one to be frightened of. "Oh that's right! Barcelona."
She was still looking at him with a mixture of worry, fear, and something else he couldn't quite place, unable to believe what she was seeing before her. Not noticing, however, he walked over to the controls, beginning to tinker with them. Listing off the time and day, he set the controls for Barcelona, finally turning back to Rose.
"Now then, what do I look like?" He paused for a few seconds, grinning at her stupidly before going on. "No, no no, no no no no no no no. No. Don't tell me. Let's see...two legs, two arms, two hands...Slight weakness in the dorsal tubercle. Hair! I'm not bald!" He seemed not to notice the fact that Rose still hadn't moved, and was looking at him as if he were an intruder, someone not to be trusted. He continued figuring himself out, always the small bit of fun offered with regeneration. "Oh—Oh! Big hair! Sideburns - I've got sideburns! Or really bad skin…Little bit thinner... That's weird. Give me time, I'll get used to it…I... have got... a mole. I can feel it. Between my shoulder blades, there's a mole. That's all right. Love the mole…Go on then, tell me. What do you think?"
She stared at him for a few seconds, still trying to stay away from him, as though he was something to hide from. "Who are you?"
His hearts fell. All those adventures, even watching him change right in front of her, and still she believed that he was an imposter, someone to be feared. She sounded scared, and it was a tone he never wanted to hear from her again. It worried him. But, he supposed, it made sense. It was her first regeneration. "I'm the Doctor."
She shook her head, not believing him. "No. Where is he? Where's the Doctor? What have you done to him?" She was becoming very scared very quickly. He wanted to walk over and hug her, make her feel better, but he knew that with her current reaction, he couldn't do that. So he stayed where he was.
"You saw me. I-I changed. Right in front of you." He desperately wanted her to believe him. Less than five minutes ago she would not have questioned a single thing he said, but now…It was as though they had just met once again, only this time, she was frightened of him.
"I saw him sort of…explode and then…you replaced him, like a…a teleport or a transmitter or a body swap or something."
She simply did not believe what she had just seen happen before her eyes. He was used to these changes, but they scared her. He simply couldn't think of anything to say to her, to soothe her worries. Because for the first time, he was beginning to worry that she wasn't going to understand, that he would eventually have to bring her home without her ever realizing that it was him.
Walking forward and pushing him, she continued, seemingly getting her courage back. "You're not fooling me. I've seen all sorts of things…Nano genes, Gelth, Slitheen."
Still shocked, in a way, he raised his eyebrows, not affected by any of this, as he had been there right with her.
Her eyes got a bit bigger, taking his expression the wrong way. Returning subtly to a more frightened tone, she voiced what had gone through her mind as she watched him. "Are you Slitheen?"
And now she thought he was going to try to kill her. Didn't even think he was a Time Lord anymore. Maybe she would figure it out eventually, though it hurt him to see her like this. "I'm not a Slitheen."
"Send him back. I'm warning you, send the Doctor back right now!"
He just couldn't take it. He couldn't lose her, and he would if he couldn't get her to understand. She had made him better, had made him more human. And if he lost her now, he would be broken. "Rose. It's me. It's—honestly, it's me." There was a tone of urgency in his voice that he hated himself for showing. He could already tell that there was more emotion in this regeneration. But he still liked to hide his thoughts. He had learned to after losing so many. He had to keep them to himself.
She continued to stare disbelievingly, panicking. He would have to explain more, spell everything out. "I was dying. To save my own life I changed by body—every single cell, but…Still me." It was true every single time. He was still the man he had been born as, though maybe a bit more hardened from all he had loved, lost, experienced.
"You can't be."
It hurt him. She still distanced herself from him, as though he would lash out any moment. He really wanted to tell her what he thought about her, how every time he saw her he just wanted to smile and laugh with her. He searched his brain for a way to convince her that all the time they had spent together wasn't lost, and stepped forward, his eyes looking straight into hers.
"Then how could I remember this? Very first word I ever said to you. Trapped in that cellar. Surrounded by shop window dummies oh…" He looked away, realizing just how far they had come since that day. They had grown so close after he convinced her to come. He looked back at her. "Such a long time ago. I took your hand…" He grasped her hand, trying to show that he was the same man who had made the same motion in a burst of adrenaline many many months ago. Except this time, there was meaning behind the grasp. Unsaid words that he was even less attracted to saying in his ninth incarnation than in this new one, though he refused to say them even now. She glanced at their hands, and he saw a spark of belief in her eyes. "I said one word. Just one word, I said…run." He searched her face, hoping that she would accept him. Her eyes were wet, full of the worry that he had abandoned her. But he refrained from comforting her, waiting to see if she would understand.
"Doctor."
A huge grin spread across his face as relief washed over him. She understood. He didn't know if she would accept it, but she understood that it was still him. "Hello."
She sighed, almost falling backwards. He couldn't tell if she was relieved or just shocked, but she believed him. Running back to the console, flipping switches, he continued, much happier. "And we never stopped, did we?" They never had. Always running. "All across the universe. Running, running running, and that one time we had to hop! Do you remember? Hopping for our lives. Yeah? All that hopping? Remember hopping for your life? Yeah? Hop? With the…no?" She had returned to her previous post, watching him from a distance, disturbed by this change. His wild hopping had been another attempt to make her realize that he was the same man, though maybe in a different body, with the same memories, abilities. But she still looked at him as though he was a stranger, one who was trying to make her feel as though they had been close when they had truly never met.
"Can you change back?" And again his hearts sunk. She may believe him, but she wanted her Doctor. Of course she did. He was the one she trusted, the one she had risked her life with so many times. But he would never exist again.

"Do you want me to?"
"Yeah."
"Oh."
"Can you?" With all the things she had seen, of course she would think that he would be able to with some sort of technology. But she would never see him again in that form as long as she lived. He was gone, though he was still standing there.
"No." She wouldn't be able to accept that this man had replaced hers. With the distance she still kept, he worried that she would never be able to grow close to him again. "Do you want to leave?" It pained him to have to ask. But if she wanted to leave, he wouldn't want to force her to stay.
"Do you want me to leave?" She thought he was kicking her out. But it was completely the opposite. The last thing he wanted her to do was leave.
"No! But…your choice, if you want to go home." She simply stared at him, giving him the impression that she did want to leave. "Cancel Barcelona. Change to…London, the Powell Estate…ah, let's say the twenty-fourth of December. Consider it a Christmas present."
She had slowly approached him as he set the coordinates, disbelief etched on her face. He hoped that the reality of going home would sink in so that she would realize she would be leaving forever if she went now. Though he knew he could never live with that. "There."
He folded his arms, watching her. Her eyes read of sadness and dread, torn between wishing for her Doctor and wanting to go on new adventures with this one, but wanting anything but a return to her home and a domestic life. "I'm going home?"
He hoped that this was a request to stay. "Up to you. Back to your mum. It's all waiting. Fish and chips, sausage and mash, beans on toast—no! Christmas! Turkey! Although, having met your mother, nut loaf would be more appropriate."
She looked away, a small smile on her face.
A bit happier that he finally took that worry off her face, he stopped a smirk from making its way onto his. "Was that a smile?"
She wouldn't meet his eyes, finally, it seemed, realizing that this truly was her Doctor. It had taken the remembrance of her mother to make the truth of it all hit her, but all of a sudden everything felt a bit more comfortable. "No."
"That was a smile." The first one she had given him in this incarnation, the smile he had seen so many times before. But never before had he been so relieved at seeing it.
She looked at him seriously. "No it wasn't."
He grinned as he teased her. "You smiled."
"No I didn't."
He wasn't going to allow her to deny it just because the last time she had given him one was when he was a different man. He was the same. "Oh, come on all I did was change, I didn't—"

Her head snapped up as she realized something, her eyes widening as she interrupted him. "I've seen you before."
An expression of shock flitted across his features. "What?"
"I've seen you before. New Year's Eve."
He frowned. "You're sure that it was me, not some other alien?" He gave her a slight smirk, trying to reaffirm, make concrete, the fact in her mind that he was still the Doctor.
She almost grinned, before shaking her head no. "Yeah. You gave this to me." She pulled the letter out of her jacket pocket, holding it out. "Said not to open it until you told me to, but to keep it with me at all times. Thought you were drunk, but I never did open it."
He grabbed the paper, his eyebrows coming closer together as he sonic-ed the paper, folded elaborately into an envelope about the size of a note card. It was definitely something he would have done.
He looked back up at her, a confused expression still on his face. "I didn't give you anything else, did I?"
She looked as though she was thinking for a moment, before her eyebrows shot up and she went off to her room without a word. For now, at least, the trip home would have to wait.
He continued sonic-ing the paper, sure that he would have given himself an explanation. If he hadn't wanted her to open it until he said so...it made him all the more sure that there would have been something else. He couldn't open it while she was gone and then refold it; he had no idea what was contained, and the information kept inside could contain spoilers.
A few minutes passed before she came back, with a much simpler letter in her hand. This one obviously had been done much quicker-he must have thought that if he was sending it to himself, presentation didn't matter. And it didn't. "He said to give this one to you. That's all. It's why I forgot about it, I suppose."
He nodded, taking the paper silently from her. "I'll be back."
If she couldn't know what was in the other, surely this was meant only for his eyes. He didn't know why he had wanted to communicate with his past self; it would normally be only in times of complete emergency. But if the message had been able to wait who knew how long, it must not be that urgent.
He made his way to his room, where he opened the letter.
Inside was an explanation of when and where this letter had been passed on, and the fact that he had been about to regenerate when he wrote it—funny, considering what he had just gone through. It told what was contained in Rose's letter, and when he should instruct her to open it.

No matter what you do to prevent it, there will be a day when she must leave. Make the most of the time between now and then, because someday it will be ripped away, and you will have wished you could have more. But you won't be able to. Even when it seems she can come back, it becomes impossible, because it seems that no matter what, after everything you have gone through you are not going to be able to keep everything.
And then there will come a day where you wish you could have said something, something you desire to tell her even now. But I know you won't, didn't. And then you didn't have the chance, because you needed her to accept change, and telling her would only make it worse.
Her letter tells her what you wish she could know. Tell her not to open it unless you are forced apart and there is no chance of finding each other again, or unless you tell her to, in which case you will be about to die. For real. Do not tell her that this is inevitable. Let her believe she will stay forever, because you both wish it.
Forget this letter until it is needed.
Make the most of your time together.
Doctor

He shook his head lightly. The letter had brought out his biggest fears, told him they would come to be a reality soon enough. But now when he went back, he had to act as if he didn't know anything, that this letter contained only meaningless nonsense, though he half wanted to throw it across the room and tell it that it didn't know anything. But he knew that it carried only truth. No matter how hard the two of them tried to prevent it, catastrophe would come eventually—at least, as long as she stayed with him after this. She still hadn't reaffirmed whether she wanted to stay or not. He couldn't let her know how much bad news this set of letters brought them. So, putting on the expression he always wore when he was trying to hide the fact that he was having a hard time getting over something, he walked back out to the control room, where Rose was still waiting. Hearing him enter, she looked up from the envelope that she had been studying, trying to learn something about it from the exterior.
"What is it?"
"It's something important. But you cannot open it until I tell you to, or until a time comes where I am not there to tell you to open it."
"But that's not gonna happen. I'm staying here forever, yeah?"
Despite all that he had learned in the letter, he grinned at her as he finally realized that she planned on staying with him despite his new form. "Yeah."
A few seconds passed before he continued. He had to ensure that her curiosity would not get the better of her. "But you cannot open until I tell you to. Opening it early could be catastrophic. Okay?"
She nodded. "Promise."
But she could see through his agreement. She knew this forever wouldn't last, as hard as both of them may try to keep it. As much of a shock as she had had earlier, she never would have left him. He was her Doctor, whether or not he had cheated death. But now she knew what had happened on New Years. He had come alone, without her by his side. In fact, he hadn't had anyone by his side. He wouldn't come back to visit her if she was with him, unless the TARDIS was pulled there by some unknown force. She knew there was only one reason that he would come back to her past without her. He didn't have to hide what the letter had told him, because from what she could tell, both of them now knew that it would come eventually. She had to make the most of their time together before the inevitable eventually happened.

The TARDIS lurched as the Doctor began sounding extremely sick, unable to finish a sentence he had just begun. "Uh oh."
She slowly moved towards him, feeling a bit closer to him now that she knew who he was for sure. She still appreciated him as her Doctor. She just had to get used to the change. "Uh…Are you alright? What's that?" A golden string had erupted from his mouth, filling the air of the TARDIS.
"Oh, the change is going a bit wrong, I'm all—" He was cut off again by the pain of the regeneration's problems, falling to the floor.
She searched her mind quickly for a way to help him. "Look, maybe we should go back. Let's go and find Captain Jack, he'd know what to do."
"Ahh, he's busy! He's got plenty to do rebuilding the Earth! Ohh…I haven't used this one in years." He hadn't needed to speed home in years. It was generally only to be used in emergencies, as it could have nasty side effects for the TARDIS.

They both were knocked to the side as the TARDIS lurched forward. "What're you doing?" She asked him, frightened.

"Putting on a bit of speed! That's it! My beautiful ship, come on, faster! That a girl! Faster! Want to break the time vortex?"

He was scaring her, but he didn't seem to realize it. "Stop it!"
"Ah, don't be so dull—let's have a bit of fun! Let's rip through that vortex!" He was growing reckless and thoughtless due to the problems with this regeneration. He couldn't help himself—he had no control over this sudden change in thoughts and feelings. But suddenly he was able to control himself for a few seconds as he stared into her eyes over the console. "Regeneration's going wrong. I can't stop myself. Ahh, my head." And he lost control again.
They were going to crash and nothing was going to stop them.