a/n: This started out as a slightly cracky idea which quickly turned into something else entirely. Dedicated to Bittie752, who read a rough draft of this and gave me the encouragement to finish it, in thanks for convincing me (very indirectly and gently) not to pull a third of my stories. Thank you, sweetie.

The Past is Prologue

This wasn't her worst nightmare. That was the pocket universe that had been created around Donna Noble.

When she'd materialized—shoved into existence through the barrier between universes by the dimension cannon—on that dark, Central London street in that universe, still slightly disoriented from her journey and believing she was finally home, she'd been full of hope, excited at the prospect that she'd finally see him again—

only to find the area cordoned off—

the street filled with gawkers and yellow-vested police and UNIT soldiers—

and a dead body on a gurney, the sonic screwdriver lying forlornly on the ground beneath it.

That was her worst nightmare, the waking one of the Doctor's death that she was only able to break free from with Donna's help.

No, as she walked back out of the TARDIS onto the cold, desolate beach in Norway in the parallel world and, heart sinking, realized the Doctor intended to leave her there, she knew that this wasn't her worst nightmare. It was, however, her greatest fear. It was a fear she'd had ever since she'd met Sarah Jane Smith, the fear that he would leave her behind. Abandon her, just as he had abandoned all his other companions in the past.

All her work to get back to the Doctor had been for nothing, said a little voice inside her head. All the arguing with scientists and soldiers and bureaucrats over procedure and funding and proper paperwork…

All the studying, all the research to adapt alien tech to pinprick the boundary between universes without causing a cascade failure, only to have it work on its own as the stars went out…

All the trips to dead and dying worlds, searching in vain in parallel after parallel…

All the fights with her mother, who she knew only wanted her safe…

No, it was not for nothing, she reminded herself firmly. Never for nothing. Again the Doctor had saved the universe, actually the entirety of the multiverse this time, and her warning had played some small part in it.

But it was at a cost.

That's me when we first met.

As they stood on the rocky shore in the parallel Norway, with the clouds racing across the sky and the wind whipping across the bay, the Doctor was saying that the cost was him. His twin self, wearing a blue suit with a collarless shirt rather than brown pinstripes, but still looking so heartbreakingly familiar.

But the Doctor was wrong, she thought. The cost wasn't a person. The cost was her heart.

The new Doctor—what did she call him? A spare? A clone? A, what was it he'd said, a meta-something-or-other?—looking so solemn and serious (so uncertain, so un-Doctorly) was telling her he was part human, would age like a human. Would spend his life at her side, together.

If she wanted.

She stepped forward and placed her hand on his chest. And as she felt the beats of his singular heart under her fingertips, she felt his breath catch, felt him lean into her touch.

She knew what the Doctor wanted, what they both wanted: for her to stay behind to play Happy Families with the man who thought the same, had all the same memories and feelings as the Doctor.

But wasn't him.

It wasn't right, because as much as this man was like the Doctor, the Doctor himself, the man she loved and whom she believed still loved her, was standing behind her, getting ready to leave her behind.

The TARDIS engines groaned, interrupting them.

"We've got to go," the Doctor said. "This reality is sealing itself off forever."

"But, it's still not right," she told him, "because the Doctor's… still you."

"And I'm him," he insisted.

Prove it, she wanted to shout. If they were truly the same person, she could stay. If they were the same person, they'd answer the same way.

But if they didn't…

"All right," she said. "Both of you, answer me this." She turned to the Doctor in brown pinstripes. "When I last stood on this beach, on the worst day of my life, what was the last thing you said to me?" He didn't answer. "Go on, say it," she insisted.

"I said, Rose Tyler," he said.

"Yeah, and how was that sentence going to end?"

He didn't hesitate. "Does it need saying?"

Crushed, she stared at him, and after a moment he looked away, as if he couldn't face her after what he'd just done to her.

She turned back to his doppelganger, him with the blue suit. "And you, Doctor? What was the end of that sentence?"

Even before she'd finished speaking, he began to lean forward, and then, only inches away from her ear, he stopped. For just a moment he hesitated, looking uncertainly at her, as if he were engaged in some sort of massive internal debate with himself.

Then his expression cleared. He whispered in her ear.

He said one word. Just one word. He said—

"Run."

In an accent that drifted North.

He backed away from her. She stared at him, confused. Was he telling her to run away? To leave with the other Doctor?

And he stared back, pleadingly, trying to get her to understand.

And then, as she puzzled over his answer, for the first time she saw—not his deep brown eyes and sideburns and really great hair, not his narrow nose and wonky ear and pinstriped suit—him. The man underneath the surface.

In some incredible, incomprehensible way, he was the Doctor. The same Doctor.

He was the same man. The man she'd traveled with. The man she'd first met in the basement of Henrik's department store.

The man she'd fallen in love with.

Without conscious thought, she grabbed his lapels. And as she pulled him towards her, he leaned forward to capture her lips.

Those few rare kisses she'd shared with the Doctor in the past had been lovely: soft and gentle, tender and sweet.

This one was none of those things.

This had a ferocity and hunger she'd never known before, something she'd always felt for him, but something she'd always believed him incapable of. And under his spell, everything—the beach, the TARDIS, the parallel Universe itself—fell away as she lost herself in their shared passion.

After a moment, as she slowly remembered they had an audience, she began to pull away, only to be pulled back by him. His kiss, at first so hard and demanding, shifted; he wrapped his long arms around her so tightly, he seemed desperate not to let her go. She reached up and caressed the back of his neck in a gesture both loving and comforting, and in that moment, she felt him relax.

Then she heard the TARDIS's engines.

She broke away from him, and ran.

After a few steps she stopped, staring in shock—

at the empty space where the TARDIS no longer stood.

And never would again.

It was over, a tiny voice whispered in her brain as she grieved. The dream, her fairy tale existence aboard the TARDIS was truly over.

A warm, strong hand took hers and gave it a gentle squeeze.

Sometimes all you need is a hand to hold…

That's all either of them had now, she thought. She squeezed back, running her thumb along the back of his hand.

She turned to face him, and found him looking back at her, concern written all over his face. Out of the corner of her eye she saw her mother, an equal level concern written on hers, pulling out her phone and backing away to give them privacy.

"Are you gonna be all right?" she asked.

"Me? I'm always—" he began, and then broke off. "Yes. I will be. Are you all right?"

She didn't hesitate.

"Yes." At his arched eyebrow, she added, "I will be."

His face cleared. He grinned at her.

"You know," he began, "I'm really glad that worked. I didn't exactly have a Plan B. I was afraid you wouldn't accept the real answer."

"What was the real answer?" she asked.

"Oh, but to tell you that, I need to set the mood. There I was, alone in the console room, in orbit around a dying star, and a holographic you in front of me. You were so sad, but you were so, so brave. We had mere moments left, and I believed I timed it perfectly. My answer would be the last thing said between us.

"But I'd misjudged. In boosting the signal, I had increased the rate the star was burning, accidentally shorteninghow much time we had."

"I figured that out," she told him.

"Of course you did," he said with a proud smile. "Just as you knew what the end of that sentence was. You always did. Brilliant, you are, absolutely brilliant. It never needed saying."

"But sometimes it's nice to hear," she said pointedly.

"Oh. Oh, yes. Yes, it is." He sheepishly rubbed the back of his neck and glanced over his shoulder, presumably to make certain her mother was out of earshot. She was. "I said, 'Rose Tyler'…"

After a pause that went on far too long as far as she was concerned, but was probably less than a second, she prompted, "And how was that sentence gonna end?"

He met her eyes. "Rose Tyler, I love you too. Absolutely and completely. With every fiber of my being. 100%. And I'm going to keep telling you that as long as you let me."

She grinned happily. "Well, I'm certainly not gonna stop you."

She grabbed him again and pulled him in for another kiss. When they finally broke apart, for air as much as anything else, he rested his forehead on hers.

"Rose Tyler, I love you."

"I'm never gonna get used to you saying that," she said.

"Well, I'm not gonna stop."

"Good."

He grinned again, and her heart did a little flip.

"Are you ready?"he asked.

"For what?"

"For the beginning," he said.

"I thought we already had that," she told him.

"Oh no, Rose Tyler. Our past, all our past, has been but prologue."

"Of what?" she asked.

"Of the rest of our lives."