The Doctor leaves Martha to sleep in the TARDIS, even though he knows that she'll be annoyed when she wakes up, thinking he's gone and done something important without her. Which, fair enough–it is important, and he is doing it without her, but it's for her own good. She's tired, and she's already done more for him than ever a human being has done for him before, and she has earned–lots of things, most of which he can't give her, but a few hours of uninterrupted rest he can manage.
Besides, if he is perfectly honest with himself (and he is, occasionally, when he's sure no one is looking) he doesn't want an audience for this errand, especially not the sort of audience he would have in Martha Jones. She is dangerously sapient, is Martha, and not just by human standards. She has the sort of mind that can leap from A to M in a single synaptic pulse, which means that the Doctor has to be very, very careful how much information he gives her, because she still thinks he is a good man, and her good opinion means more to him than he could ever let her know. And he knows that no matter how much of himself he hides from that quick, capacious human brain of hers, there is still a very real chance that someday she is going to put two and twenty billion together---that someday soon she will look at him, and he will see the light of recognition in her eyes, and no matter how fast she runs away from him after that (and she will run, how could she not?) he will never forget what it was to be known for what he is, what he's done.
That's why he leaves her sleeping. She'd told John Smith that he didn't see her; for his own reasons, the Doctor doesn't correct her.
The TARDIS lands quietly, as though it too fears to wake her.
The Doctor never forgets a story he has been told---his is an old and weary brain, but there are rooms in it where nothing is ever lost. One story has intrigued him for a long while; now, he thinks, is the time to revisit it.
He shuts his eyes and strolls down the corridors of his memory until he finds the right door. Walking through it is like metamorphosis, rejuvenation, regeneration. At once he is younger, happier, surer of himself and his place in the universe. He had been wiser then, too; he'd known exactly where that surety came from. If he wanders just a little further inside, he will find her here, the bit of her that lingers. Yes, just there; he draws aside a panel of musty velvet curtain and sees her, in a ruffled Victorian nightdress, sitting on a tall four poster bed with her knees drawn up to her chest. She looks at him like she'd always expected to see him here, like he'd come to wake her from a nightmare, just as he used to do.
"Oh, Ace," he whispers, and because it is safe here, he perches on the edge of the bed and strokes her hair. Funny to see it hanging loose like this, she almost never wore it that way, but it fits this image of her, the lonely, lost part of herself she guarded so fiercely from prying eyes.
"Manisha was dead," she tells him, as though picking up a conversation that had only stopped for a moment. "And I was so angry. I just stood there and watched that old house, burning. I thought it would make me feel better–not destroying something, but knowing something evil was leaving the world, like it would balance out the good that died with Manisha and her family."
"I wish it worked like that," the Doctor whispers.
"It didn't help, though," she says, and he nods sadly. "Nothing helped. So I went out to that playground where those kids hung out, the white kids who did it. The police hadn't caught them yet, but I knew who they were–everyone did, they were always bragging how they were going to run all the Pakis out. I was thirteen, and they were all bigger and older, but I didn't care. I screamed at them–'you killed her,' I said. I wanted them to come after me. I wanted to fight them, to die fighting them."
The Doctor's hand seeks Ace's hand within the bedclothes and squeezes it convulsively. If he concentrates hard enough he can almost feel her, warm and solid to his fingertips.
"But when they came at me, I got scared. I hit them and kicked at them, but there were too many of them, so I started running. I didn't know where to go, I couldn't go home. I wound up in this field–a great big field, I'd never seen it before, it wasn't like anything you'd expect to find in London. And then...something weird happened."
"What happened, Ace?" he says, because that's what he'd said when she first told him this story, years and years ago.
"This...thing came out of nowhere," she says. "It was dark, and I couldn't really see, but it looked a bit like–I thought it was a scarecrow. It just came sailing towards us, and I thought it was coming for me, but it went right past me and went after them. Just this great dark shape, and I could hear the straw rustling in the darkness, and I thought it was someone in fancy dress. But then the boys started screaming. And I ran away."
The Doctor sits with Ace a little longer, because he can, because it is Ace. He lets her tell him the story again all the way through, and then he sends her back to sleep. He stands and watches her for a moment, until the sun is shining through the windows over the bed. He can bid the sun rise here, though he does it more for his own comfort than for hers.
He closes the door of the room behind him before he opens his eyes again.
There is a creaky step in the stairs leading up to the TARDIS's attic. He makes a point of avoiding it, even though he is literally miles away from Martha's room. He doesn't want her to get even a whiff of this place, lest she wander into it by mistake. The ghosts in his attic are quite real, and most of them have been here for a very long time. Some of them are hungry. The Doctor himself never comes here if he can avoid it. He has a recurring nightmare that one day someone will put him here and walk away with the key. Just another reason to keep Martha in the dark---if she ever does figure out what he is, she might get ideas. Sometimes he wonders what it says about him that he even has a place like this in his TARDIS; but he has learned that there has to be a place in the universe for monsters, and it might as well be a room with a door that locks.
When he finds what he came for he takes a shortcut (makes a shortcut, really, sometimes the TARDIS takes pity on him and gives him what he wants) back to the console room, where he opens the doors and steps outside into Perivale, 1983. It is twilight, nearing full dark, and the harvest moon is already high overhead.
The Brother follows him, docile and quiet, across the field to a spot between two stands of trees. Limp as a doll, he allows the Doctor to fix him, cruciform, to two posts, arrange his limbs and his costume, and then to speak his doom as though it had been ordained this way from the beginning of the universe. And perhaps it had---the Doctor still has much to learn about the nature of causality.
"You will stand here until England is no more," the Doctor says. "You will guard these fields, and the children who wander them. That is your task. You will discharge it faithfully, or I will return."
The Brother does not speak. The Brother will never speak again. The Doctor turns his back, walks a little way. He doesn't return to the TARDIS–not yet.
The Doctor knows exactly what he's seeing when he looks into a mirror. This body he shaped for Rose's benefit, it doesn't hide him from himself. According to the myths, Merlin had aged in reverse. One day he will be Merlin. Perhaps one day soon---it's there on his to-do list, somewhere, and it feels like the right time, he'll have to ask Martha how she'd feel about playing Nimue for a bit. But even so, that's just his body. Inside, he's a superannuated old meddler who, if he's acquired any wisdom at all, has done so at the expense of all the people who ever made the mistake of trusting him.
The Doctor has learned by experience that it's safer to shape events than to shape people. Events have their own momentum, their own dim collective consciousness, and his interference counts for less in the long run. People are a different matter. It takes a special kind of hubris to look a man or woman in the eye and stamp their soul with one's own imprimatur. Getting mixed up in the odd revolution, deposition, migration, evacuation, that's a perfectly respectable job for a confirmed meddler; the other is the business of gods, and he's never sought deification, whatever the rumors say. He's never met a trustworthy god---not to say there aren't any, but the fact that he's never met one rather proves the point, that they tend to keep their celestial paws off mortal affairs, save to grant the odd miracle. That's definitely more the Doctor's style. He has learned by experience that the larger the scale of his interference, the less damage he tends to do. There have been some significant exceptions to this rule, but it's generally a sound and reliable sort of principle. It was not, however, an inuitive one. The experiences he learned from are teaching him still, because their consequences have no end.
He can't explain this to Martha. Rose, like Ace, had found out for herself.
The Doctor has hidden himself in a stand of trees, where he waits and watches as the fields of Perivale grow dark. It's not really a field, and it's not really Perivale, more a wandering spatio-temporal displacement that appears when it's called for, a bit like old Arthur, and he really ought to see about tying up loose ends there, maybe he can sell Martha on the idea of the fancy gowns after three months of wearing a maid's uniform---but no matter. It'll sort itself out in the end.
He waits, keeping himself hidden, until Ace comes. It's an act of will just to stand there and watch the boys chasing her, but that's the thing about responsibility---once you've acquired it, there's no end to it, and it really would take a god to manage the job properly, no wonder he keeps mucking it up. He hates those boys a little bit, for sharing her proper time, for making her afraid, for breaking her heart. His Ace, brave and strong and beautiful. Would he be equal to the temptation, he wonders, if he indulged himself in a little smiting? Just a small one, for their own good---but no. There: see. He's done his job. Ace is safe. She's running, still, and he knows she'll never stop, she'll run all the way to another world until she runs right into him, and he will promise to take her far away from everything she's running from. But the lies he told her have been told already, and all that's left for him to do now is clean up after his messes.
The Doctor watches until Ace is out of sight, then turns and makes his way through the tall, waving grass---back to the TARDIS, where Martha Jones lies sleeping.
He opens the door to Martha's room quietly. She stirs but does not wake.
The problem with Martha (although it's not really Martha's problem, is it, careful how you go assigning blame, old boy) is that she still thinks he's a good man, and she wants from him what any man would be happy to give. She hasn't figured out yet that the Doctor isn't a man at all, which, given the rapidity and depth of her cognition, can only mean that he has failed to put all the facts before her. Brave, brilliant, resolute Martha Jones never shied from a fact in her life. The Doctor is toying with her, a bad habit that began with Rose, playing the sort of man who would suit this body, who would capture their interest.
He has learned from experience, and he can scent a crisis of destiny coming from a long way off by now. Martha's is on its way, and when it arrives, he knows what he should do. He hopes he will be strong enough. He thinks of Ace, who might never have been sad and lost if Fenric had not wanted to throw her into the Doctor's path. He thinks of Rose, lost to him forever. He thinks of responsibilities, of lives that would never have needed fixing if he had not broken them in the first place. He thinks of Martha, who doesn't know yet that not all the monsters in the TARDIS are locked safely in the attic.
But only a god---the proper sort, not the kind the Doctor has to lock away in the attic every so often---can overcome every temptation. And Martha Jones, with her astonishing human brain, is as tempting as she is terrifying. Because there is just a chance---a small one, but real---that when she figures it out (and she will, he can't keep hiding himself like this forever, and it won't take much, not with all the elements of epiphany already bouncing around inside her busy head) that she will not just know, but understand. Not completely, because she's human, and some things are just beyond the temporally limited, not their fault, just victims of exclusionary Time Lord politics, but a little bit, just a tiny particle of the reality of the thing, just enough---
And if he keeps playing the man with her, keeps that spark of hope alive (not exactly a chore, that, he may not be a man but she would be interesting to any sentient male with red blood, let alone two hearts to pump it) then it's possible---just possible---that even knowing, even understanding---
She might not run at all.
He might look at her one day, and she might look back, and in her eyes he may find that he is not merely seen, not merely known---but forgiven.
No human could do that. Martha Jones would have to change, in ways not even she can begin to imagine now, change so much that if she ever does leave him she will find there's no place for her in her own world anymore. Her family won't recognize her, her job won't satisfy her, her planet will look like so much future rocks and ash to her unnaturally far-seeing eyes.
But why would she leave? There would be nowhere for her to go. And he would be forgiven.
Maybe that's why she's here. Or maybe it’s well past time someone made a scarecrow out of him.
The Doctor closes the door of her room and turns away. The word drums in his head as the TARDIS engines stir to life and carry them out into the darkness.