She found it during one of her regular walks. Ever since David died, and then the four children one by one, Susan walked a lot by herself.
She was married to David for nearly 50 years and, even through he knew most of her secrets, noticed how she didn’t age like he did, they were happy. And yet still she spent much of her time waiting. Waiting for her grandfather and wondering if he would ever return as he’d promised.
And she did see him a few times in some of his different incarnations, and it was nice. But it was never the same.
And then one day she went into the woods alone, her body still tingling with the energy of the recent regeneration (the despair of missing David and the children had finally got to be too much and she had made an attempt at self-destruction) and she found the blue phone box leaning up against a tree and she knew it was not simply some mouldering artifact.
She paused at first, and then she felt the tears spring to her eyes and she ran, throwing herself against the side of the TARDIS, arms spread as for an embrace. And in spite of all her pain, she smiled because the ship was not dead. The dear old thing was still alive; she could feel it, the energy under its surface.
"Oh, you dear thing. You poor, dear thing. You’re so weak."
But if the TARDIS wasn’t dead, that must mean…
"Grandfather?" she said, barely a whisper.
Then she turned, looked all around her, crying out.
"Grandfather! Grandfather, have you come back for me?"
No answer, and so she pushed open the doors of the TARDIS and went in.
The place seemed empty. It echoed with memories, her own and Grandfather’s and those of beings Susan had never known. And behind her, the doors closed, and somehow it was like being embraced by a long lost relative. She heard, or rather felt, a voice in her head.
"Welcome home, Arkytior-called-Susan."
"You’ve remembered it all," Susan said aloud, and again the feeling of a voice in her head, a peal of soft laughter.
"And how could I forget you when you were so often in His thoughts?"
"Where is Grandfather? Or when?"
"He has been away from me for a long time. What he last said was to wait for you, to wait for his dear girl, because surely if you were anywhen, you would find me."
"I don’t know."
Suddenly the console, which had been silent and dead-looking, snapped to life. Susan rushed to it, looking at the switches and levers and lights. She knew a little about piloting a TARDIS, but only in the way it had been configured when first she had traveled with Grandfather. This was very different.
"Please, I don’t know what I’m to do. Please, take me to Grandfather. Anywhere, anywhen. I don’t care."
There was no answering voice in her head this time, but things suddenly began to work. Buttons pressed themselves, switches flipped. Then there was that familiar sensation of moving into the Vortex, the strange liberating-enclosing sense of connection with all of time and space.
Susan closed her eyes, hoping that it would work. That she would find her Grandfather, the last bit of her family, again.
The bump-thump of the landing caught her off guard, but then Susan opened her eyes at last. There was nothing but stillness and silence.
Moving cautiously to the doors, she opened them and peered out.
She couldn’t be sure of the time, but a few clues gave her the idea that this must be London.
Stepping out and looking around, Susan continued to try to get a sense of her when. A sudden crackling sound drew her attention back to the old TARDIS.
As she watched, the ship shuddered, then crumbled to dust. As it disintegrated, Susan heard the voice in her head one last time.
She knelt next to the little pile of dust and scooped a bit of it up in her hand.
"Thank you, old friend."
A few people walking by noticed the young woman in her late teens or early twenties kneeling on the pavement; most assumed she was tying her shoe or something. But then an odd, coltish-looking man in a tweed jacket passed by, straightening his bow tie as he spoke animatedly to his companion, a woman with wildly curled red-gold hair.
"Oh please, sweetie," the woman said, "he thought you were one of Herman’s Hermits."
"Yeah, well, everyone makes a mistake now and again," the man shrugged.
"I still say Ed Sullivan was brilliant."
The man laughed, a single, chopped off barking sound. But then something seemed to catch his attention, and the woman’s as well, and they turned back.
They stared at Susan. Susan stood and stared back. It continued that way for quite a while until the man, squinting hard, approached the girl.
"S’cuse me," he said, "but something…you remind me somehow…"
Staring into the man’s eyes, Susan suddenly knew him, and she smiled and frowned and started to weep all at once. She saw the man’s companion approaching them slowly, hand on hip in a way that spoke of reaching for a hold-out weapon. Might be best to say something. She looked back to the man.
For a moment he looked so confused, and his face went through, conservatively, a billion expressions in a second.
"Su…Susan? My Susan?"
The girl nodded as the Doctor looked her up and down and squinted at her again.
"What’s our family name?" he asked quietly.
"What was your mother’s name?"
"Same one I was given at birth. Arkytior."
His face twisted one last time, perhaps with uncertainty, perhaps with memories of a daughter who died so long ago.
"Oh my dear girl," he said, "can it really be you?"
Susan nodded, and that was all it took to convince him. He threw his arms about her and hugged her so tightly she thought he would never stop. And he picked her up and turned around with her and smiled to his companion as he set the girl back down.
"River! River, I want you to meet Susan. My granddaughter."
River looked only slightly astonished as she reached out and shook Susan’s hand and the Doctor spoke on.
"And Susan, my dear little Susan, I want you to meet River Song. My wife."