Sorry that this is short and has little content to it yet. The main story is still forming, and developing.
I'm just seeing how this goes.

"Who are you" Susan asked, looking directly at the Tenth Doctor, observing by his clothes that he wasn't from this era.

"Well..." he started, scratching the back of his head.

"Now, now..." interjected the First, I think I've got this one, Susan."

But the young girl hadn't finished, "Wait, was that another Police Box in the yard? That's a bit curious, don't you think, Grandfather?"

The First Doctor put an arm around her shoulder, "Don't fret child, cloaking devices are clever things."

The Tenth Doctor couldn't help himself; "What?!" the two other TARDIS travellers turned to look at him. "Sorry to interrupt, but a cloaking device? Really? Is that what we called it? It's a perception filter nowadays, I assume it's the same thing. Maybe not. You probably don't even have a name for the chameleon circuit yet either, that just fits under 'cloaking device' as well." He sighed. "You're so young, you , the ship... Very well, carry on..."

There was silence; "Who is this man, Grandfather?"

The Doctor chuckled; "Can't you see my dear? It's me, older me, but I look younger."

Susan looked shocked and then smiled. "Grandfather!"

The later incarnation waved, tentatively, "Hello Susan ." He gulped, this might be difficult.