Seeing Susan for the first time brought back a tidalwave of memories. They slammed into him with a ferocity that startled him. He had experienced this before, but never that strongly, and never with such a level of clarity.
The Doctor had never fit in. He had gone to school, but never really put his heart into it. He got bullied often, and even his family never seemed to be on the same page as him. He wanted to go off, explore, see what else was out there. But he never they wouldn't ever let him. He went off to University, but with no interest, he had failed several times. He had one more chance. He had been concidering dropping out, when he decided he knew exactly what to do.
The Doctor decided he was going to see space, just like he had always wanted. He passed his classes by a hair. His family disowned him for shame, having another cousin loomed in his place. He wasn't ready to put his plans into action, but he had no place to go. That was when he met Susan. She was a young girl, dirty and starving and living on the streets. She took him to where she had been staying. It was filthy.
He touched her hand, and that was when the most vivid flashback occurred. He knew who she was. He knew that somehow, she was of his blood. He had been loomed, and was in fact much younger than her, but he knew that she had been natural born. She was of the old race. Perhaps one of the last.
He remember trying to get her to safety. He had a nagging feeling that this was important. For one who was so old, she still looked so young, and the need to protect her took over. He decided to take her with him when he left. The next day, they made their way to the bay where the TARDIS's were kept. When they were sure the coast was clear, they ran for it. Susan snuck into a new model. The Doctor was about to follow her when a woman stopped him. "Don't take that one," she told him with a smile. "Take that one." She pointed to a nearby TARDIS, an older model. "That one will be much more fun!" Susan opened the door to the TARDIS she was in, but when he turned his head, the woman was gone.
He grabbed Susan by the hand. More flashbacks. They had to get going. "Come along Susan," he said as he lead her to the TARDIS the woman had pointed out earlier. "Let's take this one instead." Perhaps it was that the girl was relieved to have someone to take care of her, but she seemed so willing to listen to him. He felt better for having someone with him, and felt relieved that he could protect this girl. "You know," she said when she noticed him looking at her. "You look and act just like my grandfather did."
"I am your grandfather, Susan." Not sure why he said it. Just slipped out. But she smiled and threw her arms around him. "You always said you'd be back for me!"
"Let's get out of here Susan."
And leave they did. And they didn't look back.