The air went cold. Everything seemed to become still and completely, utterly silent. It was the calm before the storm, and this was going to be one awful storm. she knew it. That calm, though, settled inside her. When perhaps she should have been panicking, Rose was perfectly composed. "What do you mean?"
He looked at her, shaking his head slowly. Speaking very quietly and very fast, he said, "Somehow, don't ask me how, there's been a—a tear in time. I'm not sure exactly what happened, but what I'd guess at is that two events happened that contradicted each other, so time itself split in two to cope and not cause a paradox. Problem is, it went wrong. I'm here, and you're here, but there's another one of both of us here, too. At the same time, there's another universe right next to this one, where there's one of each of us. But if we die—that is, not us, but either of our counterparts—then the timelines will merge because there's no paradox anymore."
"That makes sense," she said. "Except that there would still be a paradox, wouldn't there? There would be two of us."
"If we're in the universe we don't die in," he said grimly, "then there won't be any of us, because that universe and everyone in it will cease to exist. If we're in the universe we do die in, there'll only be one set of us. You and me, not the other ones."
"Right," she said, still calm, not really worried. True, she didn't like the idea of not existing, but the worst her feeling could be called was concern. "Do you know what happened two ways?"
"Yes," he said. "In one universe, I woke up, like I already did. In the other, I didn't."
"Which one is which?"
"We're in the universe I woke up in."
She understood the seriousness of the situation suddenly. "In the other universe, you're still in a coma. That makes you vulnerable to your enemies, and therefore a lot more likely to die."
"Which means that this version of the universe would be gone."
"And Mum would be...?"
"I think that, while the physical form of the person here would die, their mind would merge with their counterpart's."
"But we wouldn't, because there's nothing for us to merge into. On the other side, we're dead."
"Exactly. Now, I think we should move, quickly. You're about to get here, and this tear in time is actually protecting us. If you see you, there will be a paradox, and we'll be in big trouble."
Rose blinked. "Wait, say that again." Even as calm as she was, she hadn't followed that. "If I see me because I'm almost here and I'm here--"
He shook his head. "Never mind. We have to leave."
"Right." She bolted out the door.
As she ran down the street, she thought she saw herself. She paused, watching the blonde girl curiously. Yes—it was definitely her. She'd seen that once before, but it was very strange.
"Don't stand there and stare at her, get out of here!" hissed the Doctor's voice from behind her. "Run, Rose Tyler, run!"
She did. She didn't slow down until she reached Jackie's flat. "Mum!" she called. "Mum!"
The door flew open. "Rose? What's wrong — oh!"
He leaned against the wall. "Hello, Jackie," he said, his voice cheerful though he was breathing hard and looked like he was about to fall. "Time for a quick cup of tea?"
Jackie listened as they told her the story in the shortest possible way. She looked rather afraid.
"Mum," Rose explained patiently for the third time, "when the universes merge, you won't die. You'll have the memories of yourself and the other Jackie over there."
"But what'll happen to you?"
"If we're in the universe that lives, we'll be fine. If we're in the universe that's gone, so are we, yeah? No more Rose, no more Doctor."
"So then, kill the ones that are here, and everything will be fixed, and you'll be perfectly fine."
Rose sighed. "We can't touch them, go near them, or interfere with them in any way, and that includes through another person. If either or both of our counterparts die, on either side, we can't save them. If they don't, we can't kill them. If someone plans it separately from us, it's fine. If we give someone the idea, it isn't."
Jackie was lost. Rose shook her head at the very annoyed-looking Doctor. It was hopeless. "We'd better go, before they — I — we — oh, whatever. Before the others get here."
"Yes. If we see us here, there'll be big trouble for all six of us," said the Doctor brightly. "Thanks for the tea!"
He was halfway to the door, and Rose a few steps behind him, when it happened. The lights all went out, except that couldn't be right, because the drapes were opened, and the sun couldn't just go out, could it? In the middle of the day, it was not supposed to simply go black. Except that it wasn't simply black, it was black streaked with color, blue and green and hints of red...
There was no ground. She was nowhere, whirling through nothing, everything. It hurt. She was terrified. Her hands reached for nothing, and nothing was there to reach for.
And suddenly, even though she couldn't see her own hand, she felt something. The Doctor had taken her grasping fingers and his own hand curled around hers tightly. She felt safer.
And then it was over, and they tumbled onto the living room floor of Jackie's flat.
"What," asked Rose, "was that?"
"What's all that noise out there, then?" asked Jackie's voice suddenly. "Really, you should knock before—oh, Rose, it's you. What did you—Doctor?"
Rose blinked. "Er—what did I—what?"
"Well, I was going to ask what you'd forgotten, since you only just left a minute ago, but..."
"Right," said the Doctor. "I've got a good idea what all that might have been, then."
"What what might have been?"
"Don't tell me we—I don't know, switched tracks, or something? We're in the other timeline now?"
"Other timeline? Switched tracks? Rose, what're you going on about?"
"Only explanation I can think of."
"So now we have different ones of us to avoid."
"Well, technically, you have to avoid you. I don't have to do any avoiding—oh, no, I should avoid you, too. Not a good thing if you see me walking around when I'm in a coma in the hospital, is it?"
"But—you—if—what's going on?"
"So we have to avoid me, but not you, because you're in a coma and can't see you."
"What are you two going on about?"
"And...right now, we're in the universe where we're targeted by millions of aliens."
"Oh...forgot about that." The Doctor ran his fingers through his hair and scowled. "Fantastic. Just bloody fantastic."