"Still on your own?"
"Yup. Well, no. I had this friend Martha. Martha Jones she was called. She was brilliant… And I destroyed half her life. But she's fine. She's good. She's gone."
The Doctor sat in the TARDIS with his legs up on the edge of the console, thinking back to the events of the day while Donna rested in a room that the TARDIS had arranged for her. Bumping into Donna at Adipose Industries and ultimately having her join him as his current traveling companion, he thought that the day had ended brilliantly. He has someone to travel with him again, to see the universe at his side. More importantly, he wasn't alone anymore.
For several weeks after Martha had left, he thought he would be fine by himself, ready for the next adventure without her there. But there were times when, in excitement at discovering something new or solving a rather complicated problem, the Doctor would ramble endlessly only to look up from what he was doing and see no one was there.
He conjures up an image of Martha's face in his mind, examining, losing himself in memories. His brilliant Martha Jones, off somewhere doing something brilliant, as she had often done in the past when she was still with him. Only he didn't realize that until it was too late and she had walked out of the TARDIS, a broken human. Donna had asked what became of her, and he told her, in so many words, why Martha had left. The guilt of having done wrong by Martha will always linger, playing in his mind like a nightmare in slow motion, repeating, never ceasing. Someday he will have the courage to say all that he had said to Donna to the very person who deserves to hear it herself.
"That Martha must've done you good."
"Yeah she did. Yeah, yeah, she did. She fancied me."
"Mad Martha, that one. Blind Martha. Charity Martha."
He knew Donna was right. Martha must have been a little bit mad or blind, to fall for a man like him. An alien, really; he was only ever a man once. The Doctor looks back at their time together in 1913 when she confessed to John Smith that she loved him, the Doctor who was hidden inside of him. Even when he opened the fob watch and regained his Time Lord consciousness, a part of John Smith remained with him and remembered (or absolutely refused to forget) those words that Martha had uttered. Had she meant it then? She had said outright that she loved him, hadn't she? Yet, when they were due to leave Farringham, she had retracted her words saying that she would have said anything to get him back. He had taken pleasure in knowing that she had feelings for him, but not wanting to reveal this fact to her, the Doctor chose to act indifferently towards her ever since. Time we moved on, he said. But he's not entirely certain that he has.
