The Doctor stood at the TARDIS controls, thinking as hard as he ever had in his life. Donna had disappeared into the vessel's corridors, but he was not following her, not yet. Some part of him trusted her enough to let her move unsupervised on his ship.

Another part of his mind told him that he was an idiot to trust her. That he should drop her into a stasis capsule, cryogenic suspension, anything. And another part anguished at the prospect of taking his memories from her mind - could he even do that now? After all this time? Take - what had she said - seventeen years away from her? More?

Donna returned, carrying a folding wooden chair under each arm. She didn't seem to need any help, so the Doctor just watched her as she set them up, facing each other and parallel to the control panel.

She sat down in one, and spread her hands over her knees in a quick flicking gesture, and waited.

And waited a little longer.

"Well?" she finally asked.

"Well what?" he said, watching her carefully.

She snorted amusement. "Well, we both know you're dying to take a look inside this new brain of mine. So come over here and see."

The Doctor eased closer, thinking, could he-?

"Sit," and he did, facing her. She reached out and took his hand, and planted it on the console, next to the oscillation overthruster.

"One hand on the console," she said, placing her hand there as well. She reached out with her free hand and touched his face, pressed fingertips to his temple, even as his own hand did the same. They were a mirror image, arms, crossed, and then the mirror rippled into a thousand quicksilver fragments and they were -

Inside. They seemed to hover in a vague greyness, like a fog, and things like mountains or elephants, shapes great and high, could be felt pressing blindly against them in the gloom.

"Where are we?" the Doctor wondered.

"Between," Donna said softly. "Our neural impressions are overlapping, just a little. So if we went that way," she pointed behind the Doctor, "we'd end up in your mind. But - you know what's in there."

She held out her hand, and after a long moment he took it.

"Come and see," she invited him, and they walked.

Suddenly their feet rang on steel. They were in a tunnel. He looked down on armour plating and glowing conduits; he looked overhead and saw ivory and stone arches like the throat of a great whale, but instead of baleen hanging in that great throat, the ceiling was alive with weapons. Energy cannon with legs crawled about, great black shadows fringed with thorns slithered, and blinding points of light flared and froze and flared again.

"What is all that?" he said, not taking his gaze from the ceiling.

"Defences. What, you don't think I realise that I've got a huge great gob of Time Lord knowledge in my head? There's any number of races that would trade their spines for that information. So I built these, to protect me, sleeping or waking."

The Doctor kept staring upwards. Up at the mirrors with eyes of acid, and the hovering bundles of fangs, and the-

"Come on already, you'll be here forever if you keep counting them. Come on!" She tugged at his hand, and they walked on, a little faster than was quite decorous.

The long armoured tunnel grew darker, and darker, and the Doctor's hearts grew dark as well. If this was what was inside of Donna's mind...but it probably meant that his fears of her being abducted or mind-stripped were baseless.

And then in one step, everything changed.

They were on a mountain. A high mountain, dark slate-grey stone under his feet. The air was clear and cold and the sun was hot on his back, and he looked up to see the faintest fleecing of clouds in a sky so blue it hurt the eye.

Donna was standing in front of him, hands clasped in front of her like an eager student showing off her latest project. Her hair burned like a flame, tossed by a brief breeze. "Here you are."

"I - I don't quite get this," the Doctor said, slowly turning on his heel. There was no sign of the tunnel, or of anything that reminded him of the mindscapes he was familiar with. "This is your mind?"

"No, your mind."

"My mind?"

"Your mind."

"Where?"

And she laughed, the wonderful galloping laugh that he remembered, and she laughed, "You're standin' on it!"

The Doctor looked down at the stone beneath his feet. He could sense it, sunk deep into the stony earth, its roots going down into the depths. But it was also - complexity, it was shapes and sounds, smells and memories. The grey of the stone was like thousands upon thousands of lines of overlapping text, or millions of interlaced music scores, or layers of living circuitry.

It was his mind. His mind, armoured and encapsulated in Donna's mind.

"All right," he said, gingerly shifting his weight. "Then - Donna, where are you?"

"Me?" She laughed again, and spread her arms wide. "I'm everything else, Doctor. I'm the wind, I'm the clouds, I'm the sky, I'm the sun and the moon and the stars. All together, all a part of one thing. You're one hell of a mountain, Doctor, but I'm on top!"

She did a little dance of victory, her feet flashing against the stone.

"And you – did this?" He gestured at the empty space around them. Not empty space though: all of this, these skies, the other mountains, the clouds – all part of a single mind, a mental construct that would put most others in the universe to shame. "All of this – with Davros?"

"There's something a little – off – about the way you say 'with Davros,' you know," she pointed out. "A little too much emphasis on the phrase, don't you think?"

'That man – that thing is evil, the closest thing to pure evil I have ever met." The Doctor could feel the old rage rising in him. "I can't believe that he could do anything good. That he could have you in his power for even one hour without destroying you."

He suddenly realised how much he was upsetting his companion: her own teeth were clenched, muscles standing out in her neck with fury.

"Oi! You just shut up, you," Donna seethed. "You know nothing - nothing! - of what Davros could accomplish. Because you only ever met him as a crazy old man, in constant pain, and terribly alone. Think about him, think about all he did in the state he was in, at the brink of death with every breath, and then imagine what sort of a universe he could make.

"He did – he did everything for me. He saved my life, again and again, when I was about to die from ignorance or fear or pain or just about to go up like a broken gas main. He devoted hours and days and months and years to me – just to me! Just to one little human me! Because he thought I was worth it."

"I – I'm sorry Donna, it's," he breathed heavily, consciously letting go of his anger, "look, I'm sorry. It's just, I feel like we're talking about Davros' reflection or something. His – his good twin from another dimension!"

"Oh, no," she said. "No, he wasn't the good twin, no. He was – the different twin. Wait, hang on, let's sit down."

Two soft white leather recliners faded into existence, sitting at an angle to each other. Donna sat down and the Doctor did as well, a little gingerly.

Donna pushed the footrest out and stretched, and the Doctor wanted to do just the same, just to stretch, just to relax – but he couldn't. Not until he know what had happened to Donna.

"The universe of the Eternal Davros is – well, I didn't see a whole lot of it, you know? I spent the first two years in hospital."

"Hospital?"

She laughed, pressing her head back against the chair. "Yes, there was this poor administrator, Doctor Inf, and he was furious when I showed up. Said that Eternals had no business hangin' around where doctors were working, because if something went really wrong, instead of using their brains, the doctors would just pray for a miracle."

The Doctor blinked. "He did sort of have a point there, didn't he?"

"Yeah, well, the point is, the whole time he was ranting at the foot of my bed, Esselle was behind him with a mop, going over the floor. When he finally turned round and noticed her I thought he'd faint! An Eternal, mopping the floor...But then she stood up with her mop and made it clear that the Eternals were going to be in this room, working with me, and not anywhere else. And that anyone praying for a miracle was going to get their prayers answered with a boot in the arse – possibly hers."

"These Eternals take sort of a hands-on approach to their worshippers, don't they?"

"Or foot-on, yeah. But that world, that universe – it's a world where there is – not no war, but so much less than there is here. Because any time two races start gettin' ready to brawl, the Daleks show up, and they start to critique."

"Critique?"

"You bet! 'Your cannon placement is sub-optimal' and 'You have abandoned diplomatic solutions too quickly' and on and on, it's really funny when you think about it. And if they can manage to brush off the Daleks' words, then the Daleks tell 'em, go ahead. Go ahead and fight. But we fight the winners."

The Doctor swallowed. "Yeah, I can see how that would be a deterrent...But why, Donna?" The Doctor leaned forward. "Why would Davros change the world, change the universe, into anything but some crazed distortion of Skaro itself?"

"Because," Donna tapped her finger with one chin, looking thoughtful, then smiled at him.

"Because it's fun," she said simply. "Because he can see what the universe would be like if he controlled every planet, every race, and it's damn dull. Much more fun just to let everything rattle along on its own, with just the occasional clean-up when some species gets much too big for its britches."

"But now Dav - the half-Davros - is free in this universe, with the Daleks determined that he rebuild them."

Donna snorted. "Well, good luck on that. That's gonna take him a long, long time. Because what the Daleks saw in Eternity was Eternal Daleks. And Daleks do not rise to Eternity by being ruthless or vicious, or by killing. There is only one way they rise, and that's by greatness of heart."

"Daleks don't have hearts, they have circulatory flagella," he noted.

"Yeah, well, I know that. You know what I mean." She shook her head in exasperation, rubbing at her temples. "What allows a Dalek to become an Eternal, to become what these Daleks saw as bein' a perfect Dalek, is something - spiritual. And poor old Dav is going to have a long, hard road to roll before he figures that out. But when he does…well, that should be really something special."

"So all this time, the man I thought was Davros – wasn't?"

"Well," Donna leaned a little closer to him, "No, I think that was the real Davros, the real one in this universe at least."

"I met Davros before, obviously. And Ravon and Nyder, on Skaro once. But – who is Esselle?" He'd never even seen a Kaled female.

Donna smiled, her throat quivering for a moment in laughter. "She's Nyder's typist."

"She's his what?" he replied, baffled.

"A temp, Doctor. She's a temp, that's all. Brought into the Bunker on Skaro to help type up interrogation reports. And she just – stayed. Made herself useful, helped Nyder in every way that he would accept help, helped Davros too, and all the while she was turning things, just a hair, just enough to change them – enough to change everything. Without her, Davros would never have transferred into a new body. Without her – they would never have translated into Eternity."

"And she was a temp?"

"Fastest typist on Skaro, I reckon," she beamed. "She started as a temp, and wound up an Eternal. Which just goes to show you that there's no point in underestimatin' the hired help."

"She wasn't just a temp, is what you are saying."

She leaned closer to him, touched her finger to her lips in a shushing gesture, and then extended her arm until her finger brushed his lips.

"Not for you to know – or to look, Doctor. No, she was a lot more than just a temp. She knew what she was getting into, right from the start. Before the start, even. But believe me, Doctor, you try to track her down and you'll hit a time lock so cold you'll shatter. They have done what they have done, Doctor. Raised the Daleks and the Kaleds to Eternity. Even thinking too loudly about what they have done could have – consequences."

"Oh yes," he imitated her in his deepest doomiest voice, "very serious consequences – oh come on Donna, you know that's just going to tempt me, now don't you?"

"I know you." She spread out her hands, palms flat over the stone that was his mind. "You'd have gone and looked, unless I warned you. This way at least I can warn you."

The Doctor looked at his feet, scuffed one foot against the stone. He had thought that a metacrisis would be an abomination, a monster doomed to burn out in instants; and instead he found Donna, whole and balanced and perfect. She knew everything he knew, and everything she was, and combined they were – she was -

"Donna Noble, I can't imagine anyone that I'd rather have as my companion."

"Yes you can," she countered. "Rose."

"Yes, but…but…"

"Doctor." She reached out and squeezed his hands between hers, and he was a little startled to feel them still human warm. "No. I'm going home. To Earth."

"But why? What could I … what could I say to make you stay?"

"Doctor," she ran her fingers through his forelock, "no, there's nothing you can say. I've grown – too apart from you. Seventeen years – and at the same time I've grown much, much too close to you. I – having me with you would end up with us both going mad, I think, talking to each other only about our shared memories until our minds wore away like chalk."

"But - Donna, could you be happy there? On Earth? One world, one tiny pre-spaceflight world?"

"Yes." She said that with some of that same clear untroubled look she had given him, when Davros had first brought her back. "Very happy. I love Earth, and I want to see it again. I want to see my mum, and my granddad, and tell them that I love them too. I want to make some money - not a fortune, but enough so that they'll be taken care of, no matter what. And then," she smiled and her eyes stared into the distance, seeing something bright and beautiful, "and then I'm going to change the world. For the better. I know it."

"You know," the Doctor cleared his throat, "you know that there's no history of Earth where Donna Noble rules the world."

She rolled her eyes. "Oh, please. What a lot of bother that would be. I'd spend all my time tellin' people what to do, and chasin' them down when they disobeyed me. No, I don't want to rule the world. I just want to move it a bit in the right direction, you know? Give it a little shove? Like pinball?"

The Doctor regretfully put aside the image of Donna Noble tilting the planet on her pelvis, and only said, "You're too young for pinball."

"Never. Besides, I've spent how much time flying around in this living pinball machine of yours?"

The Doctor's mouth opened wide. "The TARDIS is not a pinball machine!"