She and the Doctor were walking back to the Tardis.

“And what’s that?” he asked with a smile.

“Why you really changed your mind.”

“Changed my mind about what?”

“About coming with us on the Cyberman spaceship.” She looked at him as she said it; she wanted to see the expression on his face.

His smile drooped, but only momentarily.

“The War Brain started singing; I told you,” he said.

Rose shook her head. “I don’t believe you.”

He laughed. “Oh, cheers! So I’m a liar now?”

“Why did you really come with us, Doctor? You said the most important thing was to keep that thing from the Cybermen’s hands. But then you just seemed to change your mind and took it onto their spaceship, the Cybermen’s spaceship, where they had more chance of getting their hands on it than they would’ve if you’d just stayed with it on the planet and let them go. Why? What happened?”

The Doctor’s face fell. “It’s complicated.”

“So I am right, then?”

He looked straight ahead as they walked down the corridor, not making eye contact with her. “Yes. Now let’s not discuss it.”

She shrugged. “I’m just saying.”

“And you’re very observant. Have a jelly baby.”

They walked for a while longer. The Doctor led the way up some steps, and now they were on level 38, where the Tardis was. Rose chewed her lip.

“What did the War Brain say to you?” she asked.

The Doctor scowled. “What?”

“What did it say to you to make you come with us?”

He sighed. “Not this again.”

“It did say something, though. Right?”

“It said a lot of things. Most of them lies.”

“But it must have said something that you believed.”

Suddenly the Doctor stopped. “Oh dear. I think we’re lost. Shall we split up?”

She sighed. “Doctor, don’t be facetious.”

“Rose, what does facetious mean?”

“I...I don’t know. But you’re being a prize wally at the moment.”

He chuckled. “The Tardis is just down here.”

It wasn’t, Rose found, but she didn’t say anything more until they had turned the next corner and the Doctor seemed to have forgotten her nagging.

“What I figured is,” she said slowly. She wasn’t sure this made sense. “The War Brain must have told you something, must have told you what would happen if you let it stay on the planet, and it must have been worse, much worse, than what would happen if you took it off the planet and the Cybermen got it.”

The Doctor froze. “Did it say something to you too?”

Rose stopped. She shook her head. “What did it say, Doctor?”

He let out a long, defeated sigh. He gestured for them to start walking, then pocketed his hands. “Well, you’re right, Rose.”

“What did it say, Doctor?”

“It told me that another group would arrive on the planet, by chance, in about twelve thousand years time. I would of course be long dead, and they would find my remains in the cave, along with the War Brain. It would have run out of power even before I copped it, but this group would reactivate it.”

Rose nodded. “And?”

“I couldn’t let them have it.”

“Let who have it?”

He shook his head. “Doesn’t matter.”

“A group worse than the Cybermen?”

He looked her in the eyes. “Much.”

She laughed. “Then it was probably lying, Doctor. It wanted you to take it off the planet, so it concocted the ultimate scare story.”

They turned another corner. Rose recognised where they were now. This was the corridor they were in when the Cybermen chased them back.

“It wasn’t lying, Rose,” the Doctor said.

“How can you be so sure?”

“Because the group it said would come for it, they’re gone, Rose, I destroyed them, every last one of them, and I was right to do so.”

“Wow. That bad, huh?”

“That bad.”

She thought for a bit. “It could still have been lying about them.”

“No. They only exist in the past.”

She shrugged. “So?”

They reached the door to a room labelled: E38-Beta. The Doctor stopped and took out his glowing cylindrical gizmo to open the door.

“The War Brain could only predict the future,” he said.

Rose frowned. “I don’t understand.”

He sighed. “No, neither do I.”

Then he waved the device and the door opened.

Rose found the sight of the Tardis reassuring. It was exactly as they had left it, sitting between two comparably sized rectangular crates standing on end. They were both stamped with a wolf’s-head logo, but Rose didn’t pay that much attention.

“After you,” the Doctor said, waving an open palm.

She smiled and went into the dark bay. She fumbled in her pocket for her key to the Tardis. When she pulled it out on its chain, it was dripping wet.

“Water doesn’t hurt the Tardis, does it?” she asked.

“The Tardis doesn’t feel pain, Rose.”

She scowled at his condescension. “Neither did the War Brain.”

“Though thankfully that’s where the similarities end.” He took out his own key, which was mysteriously dry, and opened the door himself.

Rose frowned and went into the Tardis.

Inside, the Doctor’s bizarre time and space machine (the acronym stood for Time And Relative Dimensions In Space) was bigger than it was on the outside. Rose went into the large, arched control room and perched herself by the console.

The Doctor dropped the War Brain inside the door.

“Actually, Doctor, I think they do have things in common,” she said.

He swung his key on its chain around his forefinger as he went up to the busy console at the centre of the control room. “Hmm?”

“The Tardis and the War Brain.”

He frowned at her as he started fiddling with the controls. “Nonsense. If you want to change clothes, go down the corridor, past the bins. Remember?”

“I’m serious, Doctor.”

“I always get nervous when you’re serious, Rose.” He pulled a few levers that didn’t seem to do anything and turned a few dials.

An odd wheezing sound resonated throughout the Tardis.

He lifted his thumb. “First difference, the Tardis isn’t insane.” He lifted his first finger. “Second difference.” He paused to think, frowned. “Actually, I think the War Brain had a more pronounced sense of humour than the Tardis.”

Rose sighed impatiently. “I mean, it’s like you were saying when we arrived, Doctor, the Tardis goes wherever and whenever it wants.”

“Yes, and it’s true.” He nodded.

“But don’t you think it’s funny, then, that no matter what co-ordinates you set, wherever and whenever the Tardis takes you, it’s always precisely where you’re needed most, where there’s some stupid apes who need your help?”

The Doctor’s hands fell from the console.

“It knows where you’re needed before you are, Doctor.” Rose crossed her arms and beamed. “It can see the future just as well as the War Brain could.”

A smile reached the Doctor’s lips. He shook his head.

“What? Well, I thought it was a good theory.”

“Oh, it is, Rose,” he said softly, turning to gaze upon her face. “It’s just that in nine hundred years of doing this, in almost a millennium of being carted around time and space by this thing, I’ve never once thought of that.”

Rose broke into a big grin. “This is why you need me.”

From deep within the Tardis came a satisfied groan.

The Doctor broke off from staring at Rose and looked over the controls like he was picking out a pet cat. “So, where do you want to go next?”

She chuckled. “You know, not that it’ll make much difference, but if there’s anyone that needs help near a shoe shop, I hope the Tardis puts them next on the list.”

Then she lifted up her bare foot, as if the Tardis could see.

The Doctor roared with laughter, then rammed a tall lever forward.

A throbbing blue light lit up the dark interior of E38-Beta. A strange, strained wheezing sound echoed in a room not big or empty enough for an echo. After a few seconds, the blue light swelled to its peak and began to fade, and by the time the last wheeze echoed, the blue police public call box had dematerialised. There had been nobody to see it arrive. There was nobody to see it leave.

NOTES:
I'm very satisfied with this short and talky final chapter. I think it brings the story full circle, to Rose understanding why the Tardis is the way it is when she started out lambasting it. I think I might have depicted Rose as being a bit thick for a laugh, and used her as the screaming damsel archetype more than once, so having her reach these conclusions that the scatterbrained Doctor failed to in 900 years makes up for that. She was always meant to be street smart rather than book smart. Explaining the Tardis might seem like a cotentious thing to do, but then, this is still just a plausible theory. But that also ties it into the "Doctor Who" mythos as a whole, seeing as the Tardis has been landing in the wrong place ever since 1963.

As for the group the War Brain warned the Doctor would find it 12,000 years later, they are of course the Daleks. You don't really need to know who or what they are, but I suppose the extra level's there if you do. This also ties it into the whole Time War theme that cropped up in the Christopher Ecclestone season. I wasn't able to slip the words "Bad Wolf" into it anywhere, though Rose does see a wolf-shaped logo on the crates in bay E38-Beta.