Title: The Reason
Rating: PG
Spoilers: S4 casting spoiler
Teaser: The Doctor's spotted Rose with Donna, but why is she running away?
Author's Note 1: This is part of the first Finish Me Fic Challenge, so it's important you read this short ditty first: http://community dot livejournal dot com/teadis/26957 dot html#cutid1

The Doctor pulled away from Rose a bit and looked at her in confusion, while the crying in the background ceased after a moment. For her part, she looked incredibly guilty, biting her lip like she only rarely did. Not sure where to start, the Doctor picked the first thing that had confused him that day.

"Why were you running away?" he asked. Did his voice just crack? Did he care?

"Because I can't stay," Rose answered, looking torn. The Doctor began to open his mouth to press her further, but he saw her take a deep breath and grip his arms a little tighter. She was going to continue on her own.

"Y'see… I'm sorry; I'm so sorry, but I lied to you on the beach." At that admission, the Doctor's brain seized. The most notable thing she'd told him on the beach was… but surely, she wouldn't be back if she didn't…? Luckily, Rose caught his vaguely horrified expression and saw the error.

"No! Doctor, I loved you. I love you. That wasn't the lie," she corrected him hastily.

His brain being slow on the restart, he sputtered back, "But… but then what?" Rose just looked at him with impossibly wide eyes, then looked away.

"Come on." She pulled him over to Donna who, for once, was being utterly silent. One hand still gripped in his, she plucked a rectangular device from Donna's arms and handed it to the Doctor. He stared at it dumbly for a moment before realizing it was a digital picture frame. Complete with short video and sound bites, apparently, because there was a giant play arrow in the middle of the screen and the words, "Ian's First Day," at the top. He hit the arrow to be greeted with several seconds of a baby squalling in a pram.

"He was born with black hair, as you can see, but that's fallen out now. Brown, like yours, it is," Rose explained with a smile, tracing the baby's face lightly with her finger.

"This is…" The Doctor's brain was back to genius speed, and it was all making sense again. "You lied; you said your mum was pregnant. But it was you. Oh, Rose, I'm…" The enormity of what he'd done hit him. Left heartbroken and pregnant with an alien baby in an unfamiliar universe. He was lucky Jackie didn't tear open reality to beat him to death with one of her fruitcakes.

"No, don't be." Rose stepped in front of him and pulled his face down to look into his lost, distant eyes. He complied and focused on her.

Rose's words came out in a tumble, "We didn't know. And if we did, what would we have done differently? I just couldn't bring myself to tell you on the beach because I knew you'd be guilty. But then I had a chance to come to this universe, and I had to let you know. Let you see how beautiful he is, a piece of you and me. He's two now; those pictures go right up to yesterday. I just thought… you should see what your son looks like." She fell silent. Then, apparently, Donna couldn't help herself any longer.

"No wonder you're so skinny; running after the spawn of that one must take Olympic stamina." Rose turned to look at her and had to laugh. It wasn't the irrepressible giggling or unrestrained laughter he so sorely missed from her, tainted by the emotionality of the situation, but it was still intrinsically Rose. Even the shadow of her former glee made him smile and, before he knew what he was doing, he leaned in and kissed her. There was no hesitancy in her response; she gave back fiercely what she took. For a few moments, there was nothing else, just lips and tongue and teeth crying out with each caress, each nip, I missed you, god, I missed you, I love-

They finally parted, breathless. Now Rose's smile was less forced.

"It's been a long time since I've been that thoroughly snogged," she informed him, smiling a little wider.

"Me too. But why can't you stay? Don't you want to?" The Doctor regretted the hushed words immediately; Rose's brow furrowed while her mouth dropped open. She was clearly insulted but continued to speak with an even tone.

"I… I can't even express how much I want to stay with you. But the technology I used to get here, it wasn't designed for young children. It certainly hasn't been tested for them, and Ian is half alien besides. He can't come over here, and I can't well leave him. So that was my plan. Give you some… keepsakes and leave."

"I'd be after some child support," Donna remarked with her arms crossed, but the couple ignored her.

"How long have you got?"

"A few hours, maximum. Likely less than two. This," she explained, pulling back her sleeve to reveal a long metallic armband, "will blink red when my time's almost up."

"Can't you visit?"

"It's too unstable, holding a connection with this world. In other universes, it's not a problem, but the walls between this one and the one I came from are already so delicate. Too many times cracked and repaired." Rose spoke in a way that was clearly brokenhearted, but the slightest sparkle appeared in the Doctor's eyes.

"Right," he asserted, "two hours or less. That's still enough to get a scan. Let me study the equipment you're using. Maybe I can come over to your side. I can't make any promises, but…" Rose looked skeptic, but the tears left her eyes.

"Do you think?"

"Don't know til I try, do I?" the Doctor asked, making himself smile confidently for her. She saw through it but smiled back all the same.

"Come on, then. I missed the TARDIS; it'll be good to say hello." Then, the trio walked briskly towards the ship, and the Doctor's mind ran at a mile a minute. If this didn't work, he would have wasted his final hours with his love on fruitless science, but if it did – and it might; it was just human tech, and he was a genius – it would be fantastic.

Author's Note 2: This, I thought, would be a very good reason for Rose to have to return to the other world after Season 4. She wouldn't want to leave the Doctor again, but your kids come first, you know?