Disclaimer: I do not own Doctor Who (BBC does). I am making no profit from this story; I simply love to write.
Notes: Tenth Doctor, in the new series. Spoilers up until the end of Series 3, along with some possibly minor ones for Torchwood through the end of Series 2 . Ten/Rose.
Part One: Yardstick
Partly, he did it just to see what would happen. He threw the words away as if they were of no moment, tossed them in her face, watched her gape. "Who says I'm not? Red bicycle when you were twelve!"
Rose obliged him, staring in awe. "No!"
She did not understand, of course, but that was all right. The humans never did, and he loved them anyway. Loved them for all of their shortsightedness, for all of their wisdom, for the sheer fact that they were so alive. Rose was all of those—incurably humanly shortsighted, wise beyond her years, and vividly alive and real, all those things he'd never be again. He learns to live through her—and then she shocks him by coming closer to understanding than anyone else has, even those of his own kind.
He chooses to die for her, then. It's creation's oldest equation: a life for a life. It works out in the end, right up until he loses her, loses himself, and only then realizes that he'd come to life and learned to love. Time Lord that he is, he runs out of time, and it's all over…until it isn't.
Years pass before he can bring himself to deliver that bicycle, and it's so bittersweet. He stands in the doorway for a long while, just watching a twelve-year-old Rose Tyler sleep, thinking of the woman the child will become.
He feels time shift slightly as he places the red bicycle under the tree. Adjust. Everything is as it should be, as it will be. Time isn't linear, after all. The moment he told Rose that he'd been responsible for that shiny new bicycle, he was. She probably hadn't even received one in the memories she'd had before that offhand comment of his, but the moment it came out of his mouth, she remembered it being so.
Click. Shift. Adjust.
Never before has he so completely understood that he is the yardstick by which time measures itself. Say the words—and it is done. All that is required are his intentions, for he is the last.
He forgets that when he tells Rose that two universes would explode—forgets that if he had told her he would find a way, he will. Intentions would make the impossible possible.
So, when he intends to cross into Pete's World, intends to leave Rose Tyler a letter on her Torchwood desk, the impossible becomes possible. But, because he does not know when that possibility will come, he cannot speak his heart. Perhaps he's a coward. Perhaps he's not.
Rose—I lied. Someday, there will be a way, and I will find it. We will see each other again.
Does she know of the power in words? He hopes so, because he signs it "Your Doctor." Only now does he understand; when the TARDIS was Rose, Rose was the TARDIS. My Doctor. Both had tried to tell him, then, what neither had words to express.
He writes the letter in his mind, quietly parked on the rift in Cardiff, knowing that he will deliver it someday, hopefully after having found Rose once more. Knowing that he will use those exact words.
It is on her desk when Rose returns from Bad Wolf Bay, turning heartbreak into something both more and less painful. She wipes the tears away, and for the first time vows to go on, promising herself that she will live a fantastic life, for him. Because he's coming back, and she's never known the Doctor to break a promise. Not to her.
She'd carried that letter in her pocket for almost two years before stopping only because Rose had been afraid that such constant folding and unfolding would tear the paper apart. Then she'd started leaving it in her top desk drawer at work, where she could see it whenever she wanted—except when she was at home. That lasted until Christine found it and asked, necessitating all kinds of awkward explanations about the man she was still pining after, three years later.
Two years after that, she had taken to carrying inside her planner, tucked into a paper protector and folded just-so. She'd thought about laminating it, but really, who laminated notes from old friends, promising the impossible?
You can't, he'd said, back on that beach.
I lied, the letter said, just a short time later.
After five years, she wasn't sure which to believe. At first, Rose had expected him to pop out from around every corner, holding out his hand with the same wild grin on his face and promising to show her the universe. The letter—oh, what she wouldn't give to know how it got there!—had kept her from dropping into the bleakest depression she could imagine, but it hadn't kept her hopes from being crushed by the simple word "someday."
Except—she still believed him. She had the letter in her hand to prove that she should. It had gotten there somehow, and so would he.
Had she dated? Yes, trying to tell herself that the Doctor was her best mate, and he hadn't said the words, and if he came to get her things would be just the same as they'd always been. But it hadn't worked, and after a few years of trying, Rose had been glad of it.
She had two younger brothers, after all (Jason and Jack, the twins of terror). Who needed to get married, have kids, live a domestic life? The only fantastic life she'd ever wanted was one with him, and after a few years of trying to fight that, she simply didn't bother. It took more energy to pretend that she wanted something normal than it did to survive.
In the end, survival hadn't turned out so bad. At least her work at Torchwood was interesting, and her dad did practically own the place. Her life wasn't a bad one, and she did have a wonderful family. Things could have been much, much worse.
So she lived her life, day by day, making what difference she could. The Doctor had taught her that.
Words have power, as the Bad Wolf understands. I am the Bad Wolf. I create myself.
There are more powers at work than just time.