A/N: Not canon. Donna is still travelling with the Doctor after his clone is created.
The Doctor stood fiddling with the controls of the TARDIS. It was early by Earth time, about 3 o'clock in the morning. Donna was fast asleep somewhere on the TARDIS. It was hard to tell exactly where she was, either because her room had a funny habit of moving around or because Donna herself would fall asleep in different places. The Doctor smiled as he remembered the one time he'd found her sleeping in the closet. She'd made a bed of a bunch of dresses, including Queen Elizabeth the First's dressing gown. The Doctor laughed quietly, fiddling with the dials. Memories had a habit of plaguing him when he couldn't sleep. Sometimes good sometimes bad, but they were there. Not sleeping wasn't exactly a new thing for the Doctor. However, this waiting business was new and he most definitely did not like it. When he'd been traveling with Martha or Rose—his thoughts stuttered over her name—he'd always just plugged in coordinates and they'd be off, day or night. His companion would always come racing down the stairs, often still in her pajamas and they'd be off.
The first time the Doctor had tried that with Donna, he'd about gotten himself ripped apart by an angry, sleep-deprived ginger. That was also the last time he tried it.
So the Doctor was stuck floating through space, fiddling with the TARDIS and being bored. Nothing was happening. The place was quiet and nothing was happening. The Doctor sighed and half-heartedly banged the mallet against the TARDIS engine. Just then, the Psychic Paper dinged. The Doctor grinned as he pulled it out and flipped the top. As the message was elegantly scrawled across the paper, the Doctor's smile slowly disappeared. He dropped the paper and staggered back, falling onto the chair in the control room. He was still there when Donna found him later. She was rubbing the sleep out of her eyes and was still in her bathrobe,
"Morning, space boy." The Doctor didn't respond and Donna turned to look at him. She was more awake when she saw the vacant look in the Doctor's eyes, "What is it?" she asked.
"We're going to a wedding."
"Excellent. Whose?" said Donna, making her way to the kitchen.
"Mine." Donna spun so fast the Doctor almost missed it. She came charging up to him and poked his chest,
"And just who do you think you're marrying, sunshine?" The Doctor tilted his head to the side,
"Well, not technically speaking mine. My clone's." he said. Donna breathed a sigh of relief.
"Good. And I suppose he's marrying…?"
"Rose Tyler." said the Doctor. Donna's eyes went wide.
"Rose Tyler. As in dimension-travelling Rose Tyler?"
"Yes." said the Doctor. There was no enthusiasm in his voice. Donna sank onto the couch beside him. They said nothing for a time, the Doctor wrapped up in his own thoughts and Donna in hers.
"So when is it?" asked Donna finally.
"Doesn't matter. It's in a different dimension."
"But you've been there before, haven't you?" she said. The Doctor hesitated,
"Yes. I had to burn up a sun just to be there for a few moments. It wouldn't be worth it." he muttered. Donna gave an exasperated sigh and stood up, again poking the Doctor in the chest,
"Listen up, space boy. I don't know how they do it on Mars, but to us humans, weddings are pretty important. So, come hell or high water or a bleeding supernova, we are going to be at. That. Wedding." Donna emphasized her point by poking the Doctor with each of the last three words. He turned over on the chair, facing away from Donna.
"I don't want to go." he whined. Donna rolled her eyes,
"This isn't about you. This is about Rose. I saw you two in the street. You love that woman and now she's going to marry—"
"NOT ME!" The Doctor roared as he stood. Donna stepped back in surprise. The Doctor's suit heaved as he fairly growled, "I love Rose Tyler and now she's marrying not me. A version of me, but not me. And you want me to go to her wedding? I don't have to go and it's impossible anyway, so we will not be going!" The Doctor shouted. He sat back down on the chair and crossed his long limbs pretzel-style. Donna sat beside him scrambling a bit for words. At last, she said quietly,
"You'd go to my wedding, wouldn't you?"
"Of course." said the Doctor gruffly.
"How is it different?"
"Because…" The Doctor trailed off, trying to find a way to put it. At last, he sighed in exasperation, "Because I don't love you. It wouldn't be so bad to see you find someone you could be happy with the rest of your life. I'm sorry Donna, but it's different. I can't love you."
"Of course you love me." interrupted Donna, rolling her eyes.
"But I don't! I don't love you Donna and I can't because I just don't love you. I won't love you because…" Donna raised an eyebrow at him and the Doctor caught on, "You weren't talking about romantic love." She looked down and shook her head,
"You dumbo. You love me like a friend, just like you love every other living thing in space and time. You and I are good friends. You'd come to my wedding because we're good friends. Were you and Rose ever good friends?"
"…yes." he said begrudgingly.
"If you don't go to Rose's wedding, or at least make an appearance, she will never forgive you. Now, we're going to find some sun burning up, you're going to do some Time Lord-y magic, and we're jumping dimensions to make an appearance at that wedding. Do you understand?"
"Yes." said the Doctor, standing with a groan. Donna put her hands on her hips,
"Right. Well, you find the nearest exploding sun or whatever, and I'm going to go get dressed."
"Yes, ma'am." muttered the Doctor as Donna went up the stairs to the closet. She leaned over the railing and shouted,
"OI! Don't you sass me, young man!" The Doctor chuckled to himself as he began looking for coordinates.
Almost two hours later, Donna came down the stairs. The Doctor was on the couch again, absently spinning the screwdriver between his fingers. When he saw her, he smiled. Donna wore a red dress that flared at her knees. Paired with the black blazer, it was the right combination of professional and comfortable. Donna did a turn for good measure.
"I just might outdo the bride in this outfit. I swear, Doctor, this TARDIS has the best fashion sense. I walked into the closet and the next thing I know, it's got a rack specially marked 'Donna' and I look fantastic in everything!"
"That's the TARDIS for you. She must like you." said the Doctor. The TARDIS emitted a strange humming sound in agreement. The Doctor stepped over to the console and pulled the screen over to him, "Alright, we're just beside the sun. It should be burning up in a few minutes and then we'll hop a dimension or two to get over to Rose's watch her marry her Doctor, and be back in time for tea!" he said. Donna smiled and the Doctor only had to take one look at her to know she was seeing straight through his bluster. She stepped forward and put a hand on his arm,
"Are you okay with this?"
"I'm alright."
"Is "all right" special Timelord code for... not really all right at all?" asked Donna without a smile. The Doctor took a deep breath and slowly let it out,
"Yeah." Donna patted his arm. Outside, they heard the explosion signaling the burning of the sun. The Doctor turned to Donna and gave a small smile,
"Allons-y?"
"Allons-y."