The Void between the worlds was howling nothingness. Nothing could live there, or even die there. Things simply weren't. The Void between the worlds was Hell.
Rose Tyler crossed Hell every day, looking for a man called the Doctor.
She had been doing this since he had left her on the other side of the Void, in a world where she wasn't even supposed to exist. Rose knew that crossing the Void was dangerous, but she was unwilling to take that as a good-bye.
Not only because the Void was so much nothing that, even for the brief second she spent in it, it nearly drove her mad. Not only because these continuous trips weakened the walls that separated the universes.
Toshiko had explained to Rose about the butterfly effect. She knew she had to be careful or she might accidentally force her own universe out of existence.
After a while, they all began to blur together. The universe where the Doctor had died in the Time War, the universe where she had never traveled with him, the universe where the Doctor was held captive as another Time Lord ruled the Earth.
Rose was beginning to worry she would never find the world she had been born in.
A flash of light and a deserted alleyway, always the same clothes. Rose didn't want to be memorable, didn't want to do anything to cause any changes.
The Doctor's words were rattling around in her brain, though Tosh had said them, too. Don't wander off. Don't become part of events. Don't change anything unless you're supposed to.
Crossing the Void was supposed to be impossible. Rose Tyler was doing the impossible.
The Doctor would have been proud.
She thought about him constantly, wondered what he was doing. He would be all alone, she feared, just as he'd been before he had met her. After finding the universe where the Doctor–the wrong Doctor–didn't know her, she found her thoughts increasingly dwelling on her first Doctor.
And with her thoughts of her first Doctor came thoughts of the man they'd traveled with, the man they'd left behind. Rose missed Jack, just as she missed everything about the universe that was home. She hoped that perhaps the Doctor and Jack would find each other. The Doctor needed someone, if Rose never found him.
A flash of light and a deserted alleyway. Rose Tyler crossed Hell again.
The Void howled and the walls between the worlds buckled. Something changed, something shifted, something r i p p e d.
Rose Tyler stepped on a butterfly.
"I really don't mind, though. Come with me."
"Okay." Jack's grin lit up his features.
"What?" asked the Doctor suddenly. He blinked, but Jack and Martha did not seem to notice anything amiss.
"I'd love to come with you," said Jack, beaming.