It starts because she muses aloud that Donna Noble must be taking the other universe by storm.
They're sitting on the back porch of the Tyler mansion, her hand in his, her head on his shoulders. She feels him tense beside her as soon as she says Donna and she wonders if it was a mistake, bringing that up. They've talked about what they've left behind but not whom, and she knows that half-human or not, this Doctor keeps his cards just as close to his chest. She wonders sometimes what it must be like for him, having only her for a confidant. It's not so very different from those first days, way back when, and not for the first time she finds herself hoping she's enough to keep the loneliness at bay.
"Yeah," he says shortly, and the dark look that washes over his face reminds her far too much of when he'd once told her his entire planet burned. It's more than simply missing something – there's guilt, too, hanging heavy on his shoulders and the corners of his mouth and stealing some of the light from his eyes.
She squeezes his hand and lifts her head to look at him. "What? What's that look for, what's wrong?"
He opens and closes his mouth twice, hesitating. "Donna … she's got a Time Lord mind in a human brain. That can't happen – a human brain can't handle having all that … stuff packed into it."
She has a fleeting image of Donna beaming at them from the vault in the Crucible, speaking in technobabble too fast for Rose to understand. "But what about you? You're fine."
The Doctor shakes his head. "It's different for me, I was made to handle it. Time Lord with just enough human – but Donna, she's… well, she's human with not quite enough Time Lord." He frowns, staring intensely at a spot on the lawn. "She can't stay that way. It'll kill her."
A silence settles between them like a thick fog and Rose feels a chill crawl up her spine. "So she'll… die?" The word alone feels like a ball and chain. "Can't he do something, save her?" Even now, she finds it hard to accept that there are situations the Doctor can't fix.
"Oh, sure, 'course he can. He will."
Yet he doesn't smile, so Rose knows she'll have to ask the most important question.
He takes a deep breath and exhales slowly, leaving his eyes shut a second longer than a proper blink. He swallows, Adam's apple bobbing, and when he speaks his voice is low.
"He'll have to wipe her mind of all of it – the TARDIS, Time Lords, traveling, him, me, everything."
The picture unfolds in Rose's mind with startling clarity, a truth she doesn't want to accept laid bare on the table before her. She swallows, too.
"So she'll just… forget all of it. Saving the universe, traveling with you, everything she saw and did and was -- all of it?"
She drops his hand and runs her fingers through her hair, considering. The very idea strikes a chord in her stomach and she feels uneasy. "But that's …terrible!"
His jaw sets and she sees his defenses lock into place. "It's the only way to save her life, Rose."
"But – but you can't take that from someone," she sputters. She imagines being stripped of all that, of being just another shop girl from London, the exact sort of girl she was at nineteen – and it terrifies her. "That's – that's not fair, you can't show someone those things and then take it away. Those memories are the only thing that got me through the last three years!"
The Doctor narrows his eyes, his head whipping to the side to stare at her. "You're missing the point! It's that or death, she'd burn up –"
"Maybe that's better!" she exclaims. She can see that ever-present anger – that oncoming storm – lingering below the surface, but she doesn't back off. "I'd rather die than lose all that."
He rolls his eyes and scoffs. "No, you wouldn't, because you wouldn't miss it. You wouldn't know there was anything to miss – you would just lead a normal, happy, human life without me interfering."
Realization hits her like a bucket of cold water and she raises her eyebrows. "Oh, my God – she's not even going to have a choice, is she?"
He rolls his eyes. "There is no choice, Rose!"
She shakes her head, her lips pressing into a thin line as anger reaches a boiling point in her veins. "Why are you always doing that?"
"Doing what?" He's furious, now, too, rage rolling off him in waves.
"Thinking you know what's best for everyone!" She throws her hands up into the air. "Making people's decisions for them! You're always doing it, acting like you have the final say in everything –"
"And what would you do, Rose? Stand back and watch her die?"
Rose doesn't' answer; instead she crosses her arms over her chest and stares out into the garden. "I've seen her without you, Doctor. She wasn't happy."
She can feel his infuriated stare against the back of her head.
"That was different." He says it like a warning.
"No, it wasn't. She gave her life to bring you back before she'd even met you!" She hisses in frustration and turns to look at him. "Come on, Doctor, you've got some Donna in you – can you honestly tell me you think she'd rather go back to being a temp in Chiswick?"
He stares at her for just a second, his eyes narrowed, before he leaps to his feet and strides angrily down the stairs and out across the lawn. As she watches his back it strikes her that she's never argued with him like this, not with this face. It reminds her of leather jackets and blue eyes and she thinks that maybe they really have come full circle; maybe the Doctor in pinstripes had been right.
She sits there a moment, arms folded and glaring at the ground, fuming on Donna's behalf. It's so like him, she thinks, to assume that he knows what someone wants better than they do. She's been there too many times – the Gamestation, Canary Wharf, Darlig Ulv Stranden – and it sets an indignant fire in her chest. She imagines losing those memories, having them pulled out from under her, and her eyes burn. She remembers the Donna Noble who destroyed the Reality Bomb, the Donna Noble who threw herself in front of a truck to make sure she turned left, and really, it's just not fair.
When Rose looks up, he's stopped in the middle of the grass, hands in his pockets, staring at the sky. It hits her then that she's arguing with the wrong man, that this Doctor has never taken anything from Donna Noble except perhaps some mannerisms, and some of her anger is extinguished by pity. He looks remarkably small, standing in the middle of the field once filled by Cybermen, so she stands and crosses the grounds to meet him.
When she reaches his side, he speaks before she has a chance to.
"You're right, Rose."
His voice holds nine hundred years' worth of sorrow and she looks up to see him looking ahead at nothing, eyes empty. It reminds her of the Crucible, where she'd watched him suffer silently through Davros' taunting.
She shivers, and he smiles grimly and sighs.
"You're right, and it's selfish of him, it is – so selfish – but just… imagine. Imagine that in nine hundred years you've lost all sorts of things – family, friends, your planet, your people. Countless people have died for you, because of you, your best friend's about to be added to that list but you can stop it, you can save her -- only it'll cost you both." He inhales deeply and closes his eyes. "He couldn't just watch her die, Rose, whether she wanted it or not. He's not – I'm not – strong enough for that."
All at once she thinks she understands, though she may not agree. Her heart breaks for all of them – for Donna, stripped of the stars, for her Doctor who never even got a proper goodbye, and for the other Doctor, alone in the TARDIS out there with only guilt to keep him company.
Wordlessly she moves in front of him, wraps her arms around him and pulls him as close as she can. He shakes in her arms, something she's never felt before, and his fingers cling to the back of her shirt with such desperation she feels like she's a buoy and he's adrift at sea. She feels him bury his face in her hair and she closes her eyes, listening to the thump-thump of his lonely human heart.