When they landed (quite roughly, may I add) at the Powell Estate, with Rose still in a state of shock, the Doctor dashed outside to greet the inevitable Mickey Smith and Jaqueline Tyler. On the equally rough journey to their current destination, the newly regenerated Doctor had achieved in convincing Rose that he was, in fact, the Doctor and not some Zygon. Not that Rose would have used that word as she had never received the pleasure of meeting any. I think. Well, you never know.
Anyway, back to the point. The Doctor had managed to convince her that he was the real deal by reminding her of their first adventure together. That time in the basement. 'Run!' Him blowing up her job. Them both blowing up the lair beneath the London Eye. And finally, her agreeing to travel with him throughout all of time and space in his TARDIS. As far as she was concerned, that just so happened to be one of the best days of her life so far.
Back to the present and the Doctor promptly fainted. Great. Luckily for them, this new Doctor was light enough to be shifted inside and dumped unceremoniously onto Rose's bed.
*DWHP*
Rose was sitting with her mother and Mickey in the living room drinking tea, so there were no witnesses as to what happened next. The Doctor, still conked out on the bed, unknowingly released little wisps of some leftover regeneration energy from his mouth. This just so happened to contain fragments of a certain someone's soul. In fact, the only person that noticed this occurrence was the inhabitant himself.
Now, as he had some time to spare, and that he had no concept of time inside this stranger's head, he decided, 'Why not have a little snoop around?' Now, how to move?
*DWHP*
After slightly more time than he would ever admit, the former wizard had discovered that the Doctor's mind (obviously he wouldn't have called him that) was, essentially, a neural interface. Just think 'move' and off you go. Simple really! Anyway, after around an hour of researching this man's seemingly endless stream of knowledge, memories and mysteries (still with no sense of time whatsoever), Crouch found out some quite interesting things about this man, 'Doctor'. He learned his name or, at least, his nickname. His real name was hidden. He also learned that this amazing man was over nine hundred years old, and counting. How many horcruxes does one man need!
Back to the point, Barty discovered that many things were behind doors in the Doctor's head *cough*Rose*cough*, and there were many, many more things to come.
He then remembered earlier, before he started searching for information, when he felt a piece of his very soul just float away into nothingness. But we'll get to that later. As for now, back in reality, cake!