Thanks to Topaz_Eyes LJ and Earlwyn LJ for beta-ing
Donna knows everything. She can see the whole universe laid out before her, past and future like a map, the present a miniscule dot.
In the same way she feels the planets moving through their orbits, understands the TARDIS controls, hears the Doctor's double heartbeat even three feet away, she knows her brain is overloading.
The realisation does not come slowly—there is nowhere near enough time for that—but it does come smoothly, flowing up like a spring, seeping into her: She is going to die.
She looks at the Doctor. He is moving deliberately, as though he has all the time in the world. Donna is sure it only looks like that because she knows what he's doing; calculate the time differential, calibrate the spatial relativity, press go.
"Doctor," she says. "We haven't got much time."
"Nonsense," he declares. "This is a time machine, we'll get back when and wherever we want to."
Donna shifts her weight and crosses her arms. It feels like a headache is beginning in the back of her brain. She knows, if her brain explodes, that it will be more literal than figurative. But there will always be time to glare at the Doctor as though he is an idiot, especially when he is behaving like one.
He looks up at her suddenly, as though the weight of what is happening has only now fallen on him. Their eyes lock for a full three seconds. The earth moves approximately ninety kilometres on its journey around its sun, and then the Doctor says, "No."
The word is soft, a denial of what he knows is true rather than delight at experiencing the near impossible.
He approaches her cautiously, his eyes flickering over her face, his arms outstretched. She wants to maintain the distance between them, keep her arms crossed, but the moment he's close enough she reaches for him, her hands gripping his biceps.
"You don't have to—Donna, please, there's got to be something we can do."
His sentiments are gratifying, she thinks distantly, but entirely unhelpful. He knows what's happening to her as well as she does.
"I'll be all right," she insists. Donna can see the Doctor's uncertainty, the flicker of suspicion and denial in his eyes. They do not have identical knowledge; he does not believe her, but Donna knows that deep in her very human bones she will be all right.
"Please," she says, "It's starting to hurt."
"Donna." The Doctor's voice breaks; he's unable to mount any sort of argument. There isn't one.
"I'm a supertemp," Donna says. "I come in and do what has to be done and then leave again. Not everyone can do that. Even when I don't remember this, I'll still know I can do it; I could always do it."
The Doctor smiles, tears brightening his eyes, but not dimming his love for her.
"And anyway, Mum'll remember and she won't be so, well—" She breaks off as the pain overwhelms her. Even fighting against it hurts, but she doesn't want to scream.
She lands on the floor with a thump that echoes through her head and jars her hip. The Doctor's hands are cool on her face. She looks him in the eye and he doesn't flinch.
"Do it," she says.
She feels the memories drain away, like water down a drain. There can't be anything left, nothing that will remind her of him and tear open the hastily stitched veil between her and the knowledge of the universe.
She keeps back one bit, though. For herself she saves the knowledge that she can save the world.