Donna asks a question only Donna would ask; something that randomly came to me in class the other day. Set during Series 4, sometime after The Fires of Pompeii.

Disclaimer: I don't own Doctor Who. If I did, The Master would be tied up in my basement.

"How do you do it?"

The Doctor looked up from the TARDIS's console, eyebrows furrowed in confusion. They had just left the third moon of Petraxis 9, a lovely place if you didn't count the poisonous, man-eating plants. The local humanoid population were dying out because of these creatures, a sad but necessary course of history. Donna was looking up at him, sadness and curiosity in her eyes. Anyone else would have taken her question as perverse, but he knew better. "How do I do what, exactly?" He pulled one of the levers to his left, a bit of a smile on his face.

"Leave," she asked. Her hands were comfortably at her side. If any other human in the history of the universe were to ask this question, they would undoubtedly be fidgeting in nervousness. But not Donna.

"Leave?" he echoed, flipping a switch and trying to decide what exactly she was getting at.

"How do you leave them behind?" she clarified, boring holes in his dark eyes, leaving her impatience behind and opting for quiet sincerity.

"Them? Them as in...?"

"Everyone," she answered, leaning back against the TARDIS's railing, arms half crossed now. "You travel in this box all over time and space, and yet, everywhere seems to be covered in death. One sort or another, anyway."

"It does seem like that, yeah," he answered, turning to look at her, the smile gone from his face. He leaned back against the console and watched her expression closely.

"You've got wars, slavery, murders. How do you just... Let them all die." She stared up at him, the best temp in Chiswick. Best temp in the history of the universe if anyone ever asked his opinion about it.

"Because I have to," he answered automatically. Leave it to Donna, the woman with the big heart, to ask the most difficult questions.

"Well I know you have to. But I didn't ask that and you know it," she replied, rather perturbed.

He was silent for a moment. Oh, he knew what she really meant. He was supposed to let them die, supposed to let them live, supposed to let them fight. But why did he? Why didn't he just save them? He turned his head to the side, inclining it in the exact way that he always did when he tried to avoid an uncomfortable question. "Donna, you know I can't-"

"But that's not the point!" she said, with only slight irritation. "That's not what I mean either. Stop avoiding my question, you know what I mean."

The unspoken words hung between them like a curtain. How do you live with yourself?

That wasn't an easy question for him to answer, much less think about. He had been doing this so long that moving on was practically second nature. Oh, it hurt every time. Twice as hard, being that he had two hearts. Losing everyone he cared for, loved, admired. They were important to him, certainly, but the sheer number of beings in the universe was staggering. He could go around saving each and every last one of them, but he didn't. Time had laws that had to be obeyed. Time occasionally killed bad people. Sometimes, changing one thing would prevent the rise of a great ruler, one who would bring peace and prosperity to his people for a hundred years. He couldn't manipulate the universe to his own design; it wasn't his to change.

A thought flitted across his mind that having the universe to sculpt would be a terrible burden to bear; how could he possibly take on the decisions required to create all living beings, never mind all of history? He wasn't sure he wanted to take on the burden of deciding exactly who lived and who died. It would hurt too much, far more than it already did.

He was silent for quite a few moments. Donna never moved; she stood and waited patiently for his answer, knowing that she'd get one eventually. He took a deep, tired breath. "Sometimes telling yourself you can't fix it makes it that much easier to turn away," he answered finally. "Sometimes... you just have to let them die."

Donna nodded slowly, thinking to herself. She expected that answer, really. But that didn't make it any less painful. She still wondered how he did it at 900 years old. She dropped her arms and wrapped the Time Lord up in a hug of comfort and sympathy. A brief thought passed through her mind that she was now in that same boat.