The dying woman laid upon the hospital bed. Her family, usually ever-vigilant by her side, has gone downstairs for tea, promising her sleeping form they'd be back soon. She does seem better this morning, after all.
A nurse hurries by, intent on her own job. She rounds the corner too fast, however, and almost bumps into a man standing, looking through the window in the wall at the woman. She steps back, apologizing. Then she looks more closely at him. She's not seen him before, and by now she knows every member of the woman's family. "Are you here to see her?" she asks. He looks at her for a moment, then pulls something from the inside pocket of his jacket and holds it up. "I'm the Doctor," he says softly. The nurse shrugs and continues on, having seen his credentials, although she doesn't think this new doctor can do much for the woman.
The Doctor continues to watch the sleeping woman. Her once-red hair is now grey, and wrinkles and laugh lines show on her face. She is closer to death than anyone realises. The Doctor knows that.
And then, suddenly, it's time. He feels it, knows within the core of his being, it's now or never.
He steps into the small hospital room. This is one of the hardest things he's ever done, but he will be here for her. She will not die alone. It doesn't matter that she's grown old. It means she lived a full and happy life. He approaches the bed, and sits down on it.
"Donna."
"Donna, can you hear me?"
She opens her eyes. A man is there—she takes in his tousled brown hair, the intensity of his face, the blue suit he's wearing. She knows him. But she can't place him. "Donna, it's me. It's the Doctor"
And she remembers. Oh, how she remembers. And she starts to cry, because now she remembers, and she knows with all her being that there is only one reason he is here now. She is going to die anyway. He has waited until her last few moments to return to her. She has found her best friend only to lose him again. She remembers the Daleks and Rose and Jack and Martha and being the woman who had something on her back. She remembers all the times the Doctor told her she was worth something, that she was important. And it hurts, oh, how it hurts, but he's holding her, hugging her, and they're both crying.
And she dies with tears on her cheeks in her best friend's arms, and he holds her for a few more minutes, until his tears have dried, to be replaced by the look he wore when he returned her home so many years ago—the look of a man who has lost more than anyone can imagine.
He gently lays her back down on the bed, and walks out of the hospital. On his way by, he sees her children: Rose, Martha, and Jack. Children perhaps named by some subconscious thought remembering that these were people worth naming children after. They are going up to see her, unaware of her death until a nurse stops them at the top of the stairs. The Doctor keeps moving; his pain is the only pain he can deal with right now.
And he closes the door of the TARDIS behind him, and twists a few dials, unsure of where he's going, but the where is not so important as the actual going…