A/N: Catalyst: (n.) A person or thing that precipitates an event or change. All rights of Doctor Who belong to the BBC. I own nothing.

We all change when you think about it. We're all different people all throughout our lives. And, that's okay, that's good, you've got to keep moving, so long as you remember all the people you used to be... I will not forget one line of this. Not one day. I swear. I will always remember when the Doctor was me.

All things change in time. All things pass away. We cannot stop the passage of time or even run from it, though there are those who would try. It is strange and frightening, this change. But, what if for one moment we realized that we were the catalyst for our own change. Might it not be so frightening then?

Catalyst

"I spent all that time trying to find you. I'm not going back now."

"But you've got to. Because we saved the universe, but at a cost. And, the cost is him."

Rose could not believe what she was hearing. He would leave her. Again. And, then her Doctor in blue had turned to her when she'd made that challenge. "How was that sentence going to end?"

Three little words. Just three, tiny, little words. And, her whole life changed. "I love you."

Rose awoke startled. It was just a dream. But, no, it wasn't. It was a memory, one so grand and one so foul all at the same time. A memory now three years gone. Groggily she realized someone was trying to get her attention. The Doctor was bouncing excitedly on the bed beside her. She grumbled at him, never having been a morning person.

"Come on, Rose! Get up! Don't you know what day it is?" he told her animatedly.

This was her Doctor. Her very much human, and yet still so very Gallifreyan, Doctor. The one that had been left with her on the beach that day. Rose had to think for a moment. Was something special about today? Yesterday was Tuesday, not very exciting. Did they have plans coming up? Her birthday was last month.

And, then all at once it hit her. They had been married exactly one year today. She smiled, remembering. It was such a simple ceremony and she'd nearly had to wrestle him into it. But, he'd wanted to do right by the woman he promised to spend his life with.

She smiled at him, more awake now. "It's our anniversary."

The Doctor paused and put a hand on the back of his neck. "Oh, is it?" He looked a little nervous now. "Well, that's brilliant!" A slow smile began to spread on his face. "Absolutely brilliant timing. Oh, Rose Tyler you are going to love this!"

"You forgot our anniversary," she said in mock sternness. Honestly, she did not really care, it had just been a ceremony for her mum's sake really. For Rose, a more accurate anniversary was one that would commemorate the day a blue-eyed man in a leather jacket had taken her hand and said, "Run." But that didn't mean she couldn't enjoy making her husband squirm a little.

"Well, I…" he started. "It's not that I forgot it, it's just… Oh, Rose, just wait, really, it'll be brilliant!" The manic grin was back. This man was much too awake in the morning. He was already fully dressed in a brown pin-striped suit and tie while she was happily still snuggled in nothing but the covers of their bed.

He was leaning over her, so Rose reached out and tucked a hand around his tie, tugging him closer to her. "Why're you all dressed?" she groaned to him, loosening the tie and moving on to undoing the buttons of his shirt.

"Rose, there is something I want to show you," he said, still distracted. She would have to try a bit harder.

"Oh," she purred, coyly looking up at him. "I can't wait to see."

"Outside of this room," he tried.

"We can do this wherever you like," she winked. "There are lots of entertaining places to try." She wrapped her hands around the back of his neck pulling him very close to her now. He hovered just over her and Rose could see she now had the Doctor's undivided attention. There was adoration in the man's eyes but Rose wanted to do one better. She wanted to see a hunger there.

She placed a kiss on his jaw. "How about you show me this surprise," she moved her mouth to the hollow of his neck, "after breakfast."

She could feel his heartbeat increase. "After breakfast…" she moved a hand down to tug at his belt, loosening it. "Is great…" he finished with a groan.

Rose smiled. "Happy anniversary, Doctor."

Satisfied, showered, and dressed, Rose made her way to the kitchen of their flat. They had spent three years living in this small space together. She was surprised the Doctor had made it that long with all the walls. But the place was very open, one room flowing into the next, and all hardwood floors. No carpet and as few walls as possible. That had been the intention.

Walking into the kitchen, she found the Doctor making her the full spread and laying it out on the table. Rose couldn't help but smile, remembering how the Doctor had been when he'd first attempted to make something edible. More than once he'd ruined her appliances attempting to make them 'more functional' according to him. There were a lot of mishaps, but in the end they usually led to take out, laughter, and snuggling together watching a movie or him reading to her. Maybe it was not their old adventures, but it was home. Anywhere with her Doctor was home.

"Good morning… again," she said, coming over to him and wrapping her arms around his waist.

He gave her a quick kiss on the nose and a happy grin. "Breakfast for two coming up. Then we can get going."

"What's got a fire under you today? Are we doing something special?" She had not planned anything for him. Rose hadn't thought it would be that big of a deal to him and that they could just have a nice day in, maybe go out to dinner later.

"It's a surprise," he reminded her.

"Alright, alright. Day's free anyhow." Most of their days were free actually. Rose had worked under Pete Tyler at Torchwood for years now, mostly as a field agent. When the Doctor arrived on this world, he had started there as well. But as of a few months ago, the TARDIS they were attempting to grow had really taken off and the Doctor resigned his position and spent most of his time working on her. Though he would occasionally go in, if his particular expertise was needed to crack a project.

Having enough money saved up, Rose had dropped down to working only part time at Torchwood, preferring to help the Doctor on their beloved time machine. She was going to be fantastic, and Rose could not wait until she was ready to fly.

Rose helped the Doctor finish preparing their breakfast and sat down with him. She told him about her latest projects at Torchwood but really things had been quiet lately. Happily she did not have anything major upcoming. Lulls in this business were difficult to come by. Her phone began to ring as they were putting dishes away.

"Hi, Mum."

Jackie's voice came over the line. "Morning, Sweetheart. Wasn't sure if the happy couple would be out of bed on their anniversary."

"Mum!" she groaned. The Doctor looked over at her concerned. Rose waved and decided to leave the rest of the dishes to him, lest he might overhear Jackie.

"It's only natural, Darling. Made it through your first year married though, haven't you? You should feel proud. It's the most difficult part… Well, then there's…" Jackie continued her rambling but Rose was lost in her own memories. They certainly had had their rough patches as she and this new Doctor got used to each other. It had taken her some time to really accept that he was her Doctor. Not a clone or a copy. Same man, same memories.

"Yes, Mum, we're very happy," Rose said, drawing her attention back to her mother.

"Now, should I be worried about any half alien grandbabies?"

"Mum!"

"Can he even have babies?"

"Mum."

"Because I was talking to Charlene the other day…"

"Mum…" Rose was really not to keen on having this conversation with the woman again. It wasn't something that she was really ready to talk about. But her mother was still going on about some conversation she had. God, she could talk.

"Oh, look, here's the Doctor. Gotta go, Mum, love you, bye!" She hung up quickly before Jackie could say any more.

The Doctor walked in looking concerned. "Everything alright?"

"Oh, yeah, just Mum being her usual self. So, what is this surprise all about?"

Excitement shone in his eyes now. "I can't wait to show you."

"I have a surprise for you too, actually," Rose teased. Though, she was nervous, not sure what he'd think of it. "And, I bet it's actually a bigger surprise than yours."

"No, no, I'm pretty sure mine will be the bigger surprise!"

Rose only smiled. "Well, lead on then. Don't keep a girl in suspense." Grabbing her hand he led the way out the door and down a single flight of stairs to the flat below theirs. Rose guessed what the Doctor's surprise was then. They had taken the flat below theirs to house the TARDIS when she got big enough. It was a safe enough place to grow her, away from the prying eyes of Torchwood or UNIT. Strictly speaking they hadn't told anyone about her.

Inserting the key, the Doctor stepped in. It was a beautiful day in May and the windows were open letting clean, fresh air in. Rose took a deep breath.

"Close your eyes," the Doctor instructed her. He put a hand on the small of her back and led her forward. She heard the familiar creak of the doors and knew exactly where she was immediately, but didn't spoil it for her part Time Lord. "Okay," he breathed. "Open them."

Looking around, Rose could not help but gasp. It was their TARDIS. Of course she had been in here before but never with the final touches all done. Their new TARDIS looked similar in shape to the old one on the inside. The column was a familiar sea green, casting a bright glow all around them. The columns still looked organic but smoother now, and white. The floor was no longer a metal grating but more clean lines with tile and rubber tread. The walls were smooth like the columns and the center console itself looked more organized but still somehow retained that grunginess the Doctor seemed to love so much.

"What do you think?" the Doctor asked.

"I love it," Rose told him giving him a broad grin.

He smiled, so happy, to have his box back and his girl with him, finally all together again. "She seems to have taken some of your design preferences to heart too, though retaining our old memories. Still a blue police box on the outside."

"I wouldn't have it any other way, Doctor." Rose smiled and made her way around the room, circling the console and viewing it from every angle. It truly was amazing. She could not believe it had been three years since she'd truly been on a living, breathing, fully grown TARDIS. Oh, her heart ached for adventure. She wanted to run with the Doctor, go anywhere, be free again.

She ran over and threw her arms around the Doctor's neck. He picked her up and spun her in a circle before setting her back down. "Welcome home!" He laid his forehead against hers, arms wrapped around her waist. The Doctor opened up his mind slightly to hers allowing her to feel his emotions, to feel is contentment holding her aboard their own TARDIS.

They stood like that for hours, or maybe it was mere moments, Rose was unsure. Finally the Doctor lifted his head and said quietly, "Would you like to see the rest of her?" He grabbed her hand and pulled her along at her nod. They passed a wardrobe, kitchen, game room, library, infirmary, and the coziest bedroom Rose could ever imagine. She could not wait to explore the rest of the ship. They hadn't even scratched the surface with those rooms. It was all so very familiar and so very new.

Returning to the control room the Doctor's eyes turned a little sad. "What is it?" she asked, placing a hand on his cheek.

He pulled out his sonic screwdriver. This one was different in design than what he'd carried when she first met him. The light was still blue and the casing still silver but it was bulkier and longer. This TARDIS had replaced his beloved little device. He used it to scan the console.

"Her life… is shortened. The process that was used to accelerate her growth cannot be stopped now. She will have a short life compared to other TARDISes."

"How long, do you think?" Any time with her was better than nothing at all as far as Rose was concerned.

"She'll probably last a hundred years? Give or take a few years. About how long I suspect you and I have." He had run diagnostics on both of their systems and found that both of their lifespans would extend beyond a normal humans, though not by too much.

"It all works out then, yeah?" she told him. "We'll have her with us, go on as many adventures as we can. Didn't you once say it wasn't how long the life is but how much life is lived in the years you did have?" Rose placed a kiss on his forehead. "We will have so many adventures. And, when we've all lived an amazing life, then we'll move on together." She intertwined her fingers with his.

The Doctor smiled. "Rose Tyler, I love you." Her breath caught a moment. She was not sure she'd ever get used to hearing him say that, but that was completely fine with her. He tucked a lock of hair behind her ear. "Where would you like to go?"

She couldn't help but laugh. "It's been far too long since you asked that question. Where should we go for our first ever trip? Is there a tradition to this?"

The Doctor turned and began running around the console in a way that was so achingly familiar. Oh, she had missed it. She had missed it so very much. He pulled levers, pushed buttons. The time rotor began to move up and down and she could hear the engines as they dematerialized. Oh, that sound. It brought tears to her eyes.

He made some final adjustment and came back over to her. He was alive. Rose had seen the Doctor happy in the past few years, but none of it could compare to this moment as he pulled her into his arms and whispered, "Thank you," in her ear.

"Maybe we should let the TARDIS decide her first destination," Rose suggested as he held her.

The Doctor was about to say something when there was a sound at the door. Rose and the Doctor looked at each other. "What was that?" she asked.

Warily he walked to the doors. "Police Public Call Box," glowed backwards above the opening. There was a sound again, almost a scraping. Rose walked to his side and he opened the door.

A small cube floated mere inches from his face. "What is that?" she asked.

The Doctor stood seeming mesmerized. "This… this is a Time Lord message."

The floating, glowing white cube was from a Time Lord? But, how?! Were there Time Lords in this world? If so, they had never met any. The Doctor was holding the device to his ear. Something changed on his face. "Who is it from?" she asked.

"Me," he breathed.