A few hours later, snuggled together beneath the duvet, the Doctor murmured to Rose, "Something's happened, hasn't it? Something that you're scared to tell me about. What is it? I know you said you wanted to wait to tell me, but…the suspense is killing me."
She sighed softly. "I s'pose it's best I tell you sooner rather than later," she acknowledged.
"You've not married someone else or something equally distressing, have you?" he joked.
Rose laughed. "Nope. Definitely not."
He nuzzled the top of her head and whispered knowingly, "It's to do with Josh's age, isn't it."
"Yeah," she breathed out unsteadily. "He's a few years older than he looks. Obviously, intellectually, he's far older as well. But he doesn't look it because he ages more slowly, so he still kind of seems like a little boy. Time Lord genes for him, I guess; I mean, you're the cleverest being in the universe, but you've still always been quite immature." She paused, and he didn't contest her words, just stared at her silently. "But, it's not just him. I'm ageing slowly too. In fact, it's even slower with me."
"Or, in fact, not at all," he whispered, shifting down to meet her gaze.
Rose bit her lip. "You think so?"
"That's what it looks like."
"Josh is fifteen, really," she said quietly. "Mum and Pete look their age; she's actually embraced the grey, you know. That's why I didn't want you to see them straight away – thought you might guess that they're in their fifties and then realise what that meant about how long we've been here. And I've been nervous about telling you, 'cos…well. I didn't know what you'd say, and I didn't really want to know what this means for me."
"Oh, Rose. I don't know what to say."
She let out a shaky breath. "But I...I'm...I'm not immortal, right? Not like Jack is. Not completely..."
"I don't know," the Doctor answered apologetically. "I'll have to let the TARDIS do some tests."
"Is it because of Josh?"
The Doctor nodded. "So it would seem. Or possibly to do with Bad Wolf. Both, most likely."
"Right. Yeah. That's what I thought."
"You see, you're human, you're not exactly equipped with everything necessary to carry a half-Gallifreyan baby to full term. Remarkably, you somehow did. Something in your biological makeup must have changed during the pregnancy, aided by the fact that you'd absorbed the Time Vortex all that time ago. I certainly didn't notice anything different about you before we were separated, so that means the alterations must have occurred further through the foetus' stage of development."
"So you're saying I'm not completely me any more?"
"You are very much you, Rose Tyler. You're simply magnificent, able to do impossible things and provide an actual real-life miracle for someone who hasn't seen such a thing in nine centuries."
"Doctor," she murmured.
"Honestly, how long do you think this will last for? Is it a permanent change? I'm supposed to be thirty-six. Will I ever look older than this? How has it altered my life expectancy?"
"I don't know. I'm sorry. As soon as we go to visit the TARDIS, we'll take a proper look."
There was a tense silence for a few moments, until the Doctor felt like he desperately needed to ask her his question:
"Would it be so bad, if it means we could have forever together?"
The corner of her mouth lifted up in a half-smile. "I want that more than anything. But you have to understand; if this means I can't die at all...yet you still can, one day, when all your regenerations have been used up...it's a reversal of how things used to be with us, and that's...that's horrible, I can't imagine being with you for all those years, centuries, then watching you go before me...it's just...I can't even..."
He reached for her hands, squeezing tightly. "I know. Let's hope that for some reason, we'll simply get to spend the rest of our lives together."
"Yeah," she offered him a watery smile.
They heard a car pull up outside. "Sounds like that's the end of our peace and quiet," he murmured with a grin.
"We'd better get dressed," she surmised, listening to her parents and Josh talking as they walked up the path to the front door.
"Mmm." He bent his head to give her a lingering kiss. "We'll sort everything out, Rose. Don't worry. Everything's going to be wonderful," he assured her.
She nodded, and flung back the duvet. "Come on then. Seize the day and all that."
He smiled. "Yes. Indeed."