Slight AU because the Doctor doesn't regenerate right after going to see Rose.
Goodbye, Donna Noble
He stumbles into the Tardis, forcing the regeneration to wait just a bit longer. He draws from the strength of his ship and feels the pain subside somewhat. Then he sets the coordinates and grips the console. His head is pounding and the movement of the Tardis nauseates him, but she lands at last and he steps out into a graveyard, eerily lit by moonlight. He finds the place he is looking for; it's a family plot. His eyes stray first to a brown gravestone. Wilfred Mott / An Old Soldier /April 8, 1920 - June 12, 2014.
Good old Wilf. What would I have done without him? Nearby he sees what he came to see: Donna Temple-Noble / Beloved Wife and Mother / October 5, 1982 - October 5, 2067. He kneels and traces the words with his finger. So she died on her birthday.
Then he stands with some difficulty and makes his way back into the Tardis, setting the controls for October the fifth. When he opens the door, he sees that it is morning in London. The sun is just rising, but already the city is rushing about. He has landed just beside the Royal Hope Hospital. A wave of pain hits him, but again he forces it to subside. He has one last thing to do. Going to see Rose was for him, but this is more for someone else; a debt he has to pay, in this body, with this face. He knows that if he goes staggering in someone will take him to the emergency room, and he won't get the chance to see her before he changes, so he goes to the front desk and plasters a smile on his face.
"Donna Temple-Noble?" His smile falters, and the receptionist nods sympathetically.
"Are you a family member?"
"No. Just ... a friend."
The girl checks the computer. "Room 1022. On the tenth floor."
"Thanks."
He takes the elevator up, with his hearts thudding against his chest. He can see regeneration energy beginning to wisp around him whenever he looks hard at any part of himself. It is with a rising feeling of panic that he steps out of the elevator and then into Donna's room.
She is lying in bed, asleep. Her once-ginger hair, now white, drifts over one shoulder. Her face is wrinkled, deeply carved with the tale of many years. Her hand hangs slightly off the bed.
"My Donna Noble," he whispers, squeezing the hand gently. Then, with the last bit of strength, the only part of him that is not engaged in keeping this body alive, he presses his fingers to her temple and opens a leak in the dam that holds back her memories.
Her eyes flutter open and she squints. "Jay? Is that you?"
"Happy Birthday, Donna." If his voice is a bit thick with tears, who can blame him?
"D-doctor?"
"Yes, Donna, it's me."
She sits up and stares into his face, fumbling for her glasses. He guides her hand to them and she puts them on. "It's you!" she shrieks. "Where've you been? I've looked for ages. No, wait. I found you again, didn't I?" Then she gasps.
"Easy now," he says, taking her hand again. "The memories are coming back."
"The Ood, they sang for us when we were leaving. And Pompeii, burning. Oh, Doctor!" Tears well up in her eyes and stream down her cheeks. "Did you ever find that stuttering bloke from the Library, eh Doctor?" Then, without warning, she pulls away from his grasp and slaps him, hard.
He reels backward, and the dam inside him almost bursts. With great effort he pushes it back, weakening himself further. Somehow, the fact that she slapped him hurts more than it should; he doesn't blame her, but still ... No matter, he doesn't have time for this.
"I'm sorry, Donna," he says, reaching for her hand. "But I couldn't just let you die. Besides," he laughs shakily. "your mum would have killed me."
"Yeah, I guess she would have." She is smiling a little now, holding onto him as if she'll never let go. "But, Doctor, how long has it been since," she hesitates. "Since the Daleks? The metacrisis?"
"About a year," he says. "But I saw you after that. I was there after the wedding. Do you remember the lottery ticket?"
"What? You mean the one that bought us a nice house and a sporty car? I've wondered for years who gave it to us." Then her eyes widen. "No! You didn't!"
He nods. "Well, I got the money for it from your dad. But I couldn't give you anything stranger; it would have been too dangerous."
"I've been dreaming about you," she whispers, wiping away a tear. "for weeks. I guess the human part of my mind was getting weaker; you were coming through."
Suddenly he doubles over with a moan.
"Doctor? Doctor, what's wrong?"
"I'm dying, Donna. Right now. Dying."
"But you can't ... Why don't you regenerate?"
"I had ... had to ... see you first." He straightens again. "I've only got a moment or two left. I don't want to regenerate in this hospital."
"Take me with you!"
"I can't. I'm not strong enough to get you back to the Tardis."
"I'm dying, too." Her voice is calm for the first time. "Look at us. Two old lonely Doctors. We're always alone in the end, aren't we?"
"Yeah. I suppose we are."
She pulls him into a hug, and he grips her tightly, then presses a kiss to the top of her head. "I'll always remember you, Donna Noble." He chokes back a groan and rises to his feet.
"Goodbye, Doctor." She is crying again, silently. He smiles, a smile full of sadness and happiness and all the weight of a Time Lord's memories, but he says nothing and turns to go.
"Say goodbye," she begs. "Just this once. You don't say it often enough."
"I can't bear to," he says.
"For me, Doctor. Let me hear you say it."
He draws a deep breath. "Goodbye, Donna Noble."
Almost as soon as he is out of the room, all the monitors start beeping wildly and her voice rises to a panicked shout. "Doctor!"
A doctor and nurse run into the room, and he gets into the elevator. The regeneration energy is forcing its way out of every pore; he can't get to the Tardis quickly enough, but somehow his legs aren't working correctly. It doesn't help that her last cry is sounding in his head. He stumbles past the receptionist and collapses outside the Tardis door. Then a hand takes his and pulls him to his feet, steadying him, and through his blurred vision he sees the Ood. As the song of the Ood begins in his mind once more, he unlocks the door and steps inside.
The End
AN: Hope you all liked this! Constructive criticism is always welcome.