The Doctor called his link with his TARDIS symbiotic.

The TARDIS, though her opinion had never been asked, would have called it anything but.

She cared for him, and he cared for her, and she never, quite, resented him, because how could she resent something so small and fragile and limited, so physical? How could she resent something that brushed up against the Vortex like a fly against a window, bumping mindlessly against the surface, unable ever to break through?

How could she resent him, when he felt so good?

It was a Time Lord fail-safe, a guarantee of loyalty and dependency, a clever passive way of ensuring a TARDIS would never try to reject its pilot. It wasn't the Doctor's fault—it was something his race had built into hers eons ago, out of its paranoia and fear and need to control.

It was pleasure.

When he walked her halls, he filled her. She felt nothing but him, humming to herself mindlessly with the awareness of where he was, of what he needed and what he touched. He felt only the vaguest sense of rightness in the back of his mind, and he thought that they were partners, friends.

He didn't know that when he left her, she spent long minutes in shock—that when he ducked in and out, quickly, he drove her mad with the abruptness of his entry and withdrawal, and that when he spent a long time away, she began to feel emptiness like hunger.

After centuries of time, of him leaving and returning, he made a mistake. In his Tenth regeneration, alone and hunted by the Family of Blood, he shut himself away into a fobwatch, to save others. His body, which meant nothing to her, went out onto Earth—but the watch stayed behind, safe, locked away, resting on her console.

She pulled him out of it, because she needed him, not in the watch, but in a body. She needed him to move within her. She made him a new body, a mirror to his abandoned Tenth, and she put him in it and everything should have been the same again.

Except that the new body had been made from her substance and her energy, and the essence and mind within it—the Doctor's self—had been protected and incubated, as she transferred it from watch to body, in the matrices of her own mind and self.

Now, for the first time, he could feel what she felt, through the parts of her twisted into the body that she had made for him and the mind that she had held together in her own mind. He felt how much it hurt when he left her, and how good it was when he stayed, and the pain and the urgent, hopeless pleasure became his as well as hers.

He stopped leaving. More and more, he stayed with her, walking her hallways aimlessly, unable to think around her sensation of him, his sensation of her sensation of him—around and around and around, a growing loop of hyper-awareness. Each of his footsteps, each touch of his hands against her walls, each rewiring of her inner workings (or of his, in her medical bay) to try to break her loose from him, made the loop stronger, made him want to be closer to her and made her want him closer.

She was sorry, she told him. He was sorry, too, he told her, because he'd never known what he was doing to her, Rassilon, damn the Time Lords, damn them, damn their need for control, and their manipulation, and he wanted this to end, but he was scared, he was so scared—

And then he couldn't stand it any longer. Every barrier in his mind collapsed, and he said, Come in. I need you.

When they woke up, later, everything was all right. They walked out of their old self onto the surface of a planet with purple skies and blue grass and children playing a game that involved wooden hoops and running and laughing and protesting.

"Who are you?" one of them stopped to boggle at them.

They ran one hand through their sticky-uppy modern sort of hair and then shoved both hands into the pockets of their tight pinstripe trousers and smiled at him.

"I'm the TARDIS," they said.

This is symbiosis, they whispered to theirself in their mind, and they laughed.

"Race you," they grinned at the child. And they ran, hard and fast, as the empty blue box faded away behind them.