I don't own Doctor Who.

This is the first in a planned series. I have different parts of this written, but it's mostly towards the end, so updating will probably be quicker towards that end, from Doomsday onwards.

Bad Wolf could see everything there ever was, everything that ever will be and everything that ever could be. The Goddess of Time looked for her own future – or, more accurately, the future of one half of her, the future of one Rose Tyler – and found her life and eventual fate, woven throughout the universe, and everything that it would cause.

…And she hated it.

She saw Rose Tyler, stranded in the alternate world, abandoned. She saw Martha Jones, always overlooked due to her predecessor. She saw Jack Harkness, left behind on the Gamestation, immortal and spending eternity, ultimately, alone.

She saw the Doctor, with a broken heart. He would find another eventually, but the pain he would go through…

Bad Wolf frowned. There was one other way.

'Jack and the Doctor are out there, right now, fighting for their lives!' Rose Tyler had shouted at her mother. The part of Bad Wolf that was the TARDIS matrix noted that she had thought of Jack first.

The part that was Rose Tyler pretended not to notice.

Rose Tyler was at a crossroads in her life when she merged with the TARDIS and became Bad Wolf. There were two ways her life could go from here. One with Jack, one with the Doctor. Which, in the end, would cause more pain?

Bad Wolf frowned, held out her hands, and changed the course of history forever.

Or, in human terms –

Rose Tyler stood on a butterfly.

Rose blinked her eyes open and sat up. She was in the TARDIS console room, and the Doctor was standing at the console. They weren't in flight yet – Rose wondered if they were still on the Gamestation.

"What happened?" she asked.

"Don't you remember?" inquired the Doctor.

"There was singing," she recalled, starting to stand up.

"That's right," said the Doctor with his funny little grin. "I sang a song and the Daleks ran away."

"I was at home." Rose frowned. "No, I wasn't, I was in the TARDIS. There was this light… I can't remember anything after that."

"Rose Tyler," said the Doctor in a sad voice, "I was going to take you so many places: Barcelona, not the city, the planet Barcelona. You'd love it – they've got dogs with no noses." He laughed, making Rose smile back at him. She glanced around.

"Where's Jack got to, then?" she asked, walking over to stand by the console.

"Imagine how many times you tell that joke a day and it's still funny," continued the Doctor, ignoring her words.

Rose frowned, but asked, "Then why can't we go?"

"We will. Maybe I will. Just not like this," replied the Doctor.

"You're not making sense."

"I might never make sense again!" exclaimed the Doctor. "I might have two heads, or no head! Imagine me with no head – and don't say that's an improvement!" He paused and looked down. "It's a bit dodgy, this process," he admitted. "You never know what you're going to end up with.

Abruptly, an orange-gold light erupted from the Doctor's torso, and he doubled over in pain. "Doctor!" shouted Rose, racing towards him.

"Stay away!" he ordered her, causing her to stop in her tracks.

"Tell me what's happening," she pleaded.

"I absorbed the entire energy of the time vortex," explained the Doctor, "and no one's meant to do that." He clenched his jaw for a moment. "Every cell in my body's dying."

"Well, can't you do something?" she demanded.

"Yeah," he replied, "I'm doing it now. Time Lords have a little trick, a way to cheat death – except I'm gonna change. And I'm not gonna see you again. Not like this. Not with this daft old face. And, before I go - "

"Don't say that!" exclaimed Rose.

"And before I go," he repeated, more forcefully, "I just want to say: you were fantastic. And you know what? So was I."

The orange light erupted from him, making him scream in pain. The light came from him like fire burning on wood, so thick that Rose could not see what was happening within to the Doctor. She longed to help him, but found herself too afraid to do so. It seemed to go on forever, but it really probably only went on for a minute before it vanished.

A new man stood in the Doctor's place. He wore the same clothes as the Doctor. He had brown hair and brown eyes, and was rather tall.

"Hello," he said. "Okay." He frowned. "I have new teeth. That's weird. So where was I? That's right!" He gave her a quirky little grin. "Barcelona!"

"Who are you?" she demanded. "Where's the Doctor? What have you done to him? And Jack – where's Jack?"

"Rose," he frowned, "it's me. It's the Doctor. You saw me. I changed – right in front of you."

"I saw him sort of – explode," she said, "then you replaced him like a teleport or a transmat or a body swap or something." She walked tentatively over to him and gave him a small shove. He rocked back on his heels. "You're not fooling me. I've seen all sorts of things: nanogenes, gelf, Slitheen." He raised one eyebrow slightly. "Oh my God," realised Rose, "are you Slitheen?"

"I'm not a Slitheen," assured the man.

"Send him back! I'm warning you, send him back!" she snapped.

"Rose, it's me. Honestly, it's me." He leaned forwards slightly, hands in his pockets.

"If you're not going to tell me where he is," snapped Rose before he could say another word, "then I'll go find him." She marched straight out of the TARDIS and on to Satellite Five. "Doctor!" she called. "Jack?"

"Rose," protested the man, following her out. Across the room, the lift opened and Jack ran out.

"Jack!" she shouted, running across the room and over to him. She swung her arms around him and pulled him into a hug. "Jack, there's this man. He's swapped with the Doctor or something. The Doctor exploded and then he was there, in the Doctor's clothes and everything, insisting he's the Doctor." Jack shifted slightly so he could look over Rose's shoulder without letting go of her.

"I thought that was a myth," said Jack in a wondering voice.

"Nope," replied the man. "Regeneration. It's a handy trick."

Jack leaned back so he could look Rose in the eye. "Rose, there's this thing. I'd heard about it, sort of, in the Time Agency. There were stories about an old woman doing it way back in the twenty third century, after the Daleks disappeared, an old woman who they found out to be a Time Lord after she regenerated."

"That'd be Susan," said the man – Doctor? – knowledgably. "She's my grand daughter," he added. "She fell in love with a man on Earth so I left her behind."

"It's what happened to the Doctor, I think."

"I…" She shook her head in disbelief.

"Rose," said the stranger. She turned around, freeing herself from Jack's arms as she did so, and looked the man in the eye. "The very first word I ever said to you. You were trapped in that basement, surrounded by Autons, so long ago now. I took your hand and said one word – just one word. I said run."

"Oh my God," she whispered. "Doctor?"

"Hello," he grinned. She slumped in disbelief. Laughing, Jack caught her and pushed her back upright.

"How does that even work?" she complained as they reboarded the TARDIS.

"Trust me," replied the Doctor, "you don't want to know." Quietly, he muttered to himself, "Next she'll be asking how time travel works."

"That's a very good question," said Rose with a smirk. Jack opened his mouth to explain but the Doctor cut him off.

"Don't tell her," he told him. "You've got to admit, it's funner this way." Jack gave a small shrug.

"True."

"You guys are horrible." Tossing her hair back over her shoulder, she flounced away down the hall. Behind her, Jack and the Doctor traded amused looks.

"Definitely funner," said Jack.

"Absolutely."