The quote at the end if from one of the historical character's actual speeches. I do not claim any rights to it. The rest is the output of my addled brain.
"Does he know?" She asked, still watching the door that had closed moments before.
The Doctor mumbled something in response.
Martha spun to face the Timelord and repeated her question, emphasizing every word, "Does… He… Know?"
The Doctor finally stopped fiddling with the TARDIS controls and after a moment met his companion's eyes. "He suspects."
"Why? What did you tell him the last time you two met?"
"I told him no."
Seeing the look of confusion on her face he elaborated. "As you so wisely deduced earlier, I have met Martin before. I ran into him a few years back in this nation's Capital. Washington, D.C. Brilliant place Washington, they have the most wonderful buildings. Grant's Tomb, spectacular! You know who's buried in Grant's Tomb?"
Martha gave him a hard look and his babbling eventually died off.
"Yes… well… due to the Altaran he helped me track down I had made him late for his pep rally. I was forced to reveal myself in order to get him to the gathering on time, so to speak."
Martha still looked confused, "What does that have to do with saying no?"
The Doctor's manic grin was displaced by a more somber visage. "When he found out I could travel in time he asked to see the future, see how it all turns out. I had to tell him no. If he had seen the future the temptation would have been too great for him to change it.
His eyes locked with Martha again. "You can't change time, ever. Remember that."
She did not know why the sadness and desperation had crept into the Doctor's voice but she nodded her agreement to his request, afraid to speak lest she interrupt his tale.
"Finally he acquiesced and agreed that it was too dangerous. There was too much of a chance of him making an offhand comment or relating something to a friend that could alter history. He did not see me again between that day and this."
"And that's why he suspects?" She asked.
The Doctor nodded his in affirmation.
"That you showed up again," she continued, "and that you have now shown him the future would make him suspect that he won't be around long enough to do anything with that knowledge."
The Doctor beamed at her deductive skills and she basked in the glory of his silent praise.
Martha's brow furrowed, "but what if he says something now about what he's seen?"
"Oh, he will. Tomorrow night.
"That's why I made sure he had the proper perspective on events."
It took her a moment but Martha eventually rolled her eyes at his obviously poor attempt at a pun.
Martin sat in quiet contemplation, as he had for most of the day, and considered what the Doctor and his young black companion had shown him. He had seen one of his own people win a victory to the highest office in the land. More than that, he had seen people of every nationality and race cheering and celebrating the man's election. Finally he had seen his long time friend, a man who stood only feet away from him right now, breaking down in tears of joy. He did not know the year or the day, but if his friend was there it must be coming fairly soon. He had looked down from the balcony the Doctor brought him to, out across the multitudes gathered to listen, and heard a black man accept the nomination for President.
As he approached the podium to speak the knowledge the Doctor granted him lifted some of the burdens he carried these past few weeks from his shoulders. His dream would exist, and some in this very room would get there.
Finally with that in mind, he addressed those gathered, "Thank you very kindly, my friends. As I listened to Ralph Abernathy in his eloquent and generous introduction and then thought about myself, I wondered who he was talking about. It's always good to have your closest friend and associate say something good about you. And Ralph is the best friend that I have in the world…"
