"Doctor, what's this?"
The Doctor looked over his shoulder at his companion. They had been taking a tour through Jack's Torchwood and were currently in their archives. Donna had apparently found something interesting.
"Hmmm... Not quite sure," he said, looking down at the device in Donna's hand. It definitely seemed futuristic and alien. Not the sort of thing he necessarily wanted to leave in Torchwood's hands, even if he did think Jack halfway knew what he was doing. His finger grazed the big button at the top of the device, the type that beings put on to demand attention for some kind of nefarious purpose.
"Doctor, what are you do - " Donna started to, trying to jerk the device away and inadvertently pressing the button herself. Then a white flash engulfed them both.
* * *
Jack Harkness was not having a good couple of weeks.
When the Doctor and his current companion, Donna Noble, arrived in Cardiff, he had been happy to see his old friend again and meet Donna. She seemed to be exactly what the Doctor needed. He had even given them a tour of the Hub. He had not thought that anything would go wrong when they went to look at the archives. Of course this was the Doctor and Jack really should have known better than to leave the Doctor alone for even a millisecond amongst the various alien machinery.
So maybe it really should not have surprised Jack when he came back from a brief consultation with Tosh over a spike in the Rift that the Doctor had pressed the wrong something or other. Because when he got back, Donna Noble somehow became a baby and the Doctor had been regressed into what Jack had later found out was his very first incarnation!
After analyzing the previously unworkable device before the Doctor and Donna had gotten their hands on it, Tosh was able to learn that it was a weapon of some sort. That should not have surprised Jack either. The weapon apparently regressed its victim all the way back to its infant years, or in the Doctor's case his very first century. The good news was that the process would eventually reverse itself, but Jack had no idea when that would be. He ordered Ianto to seal up the device in the most secure vault Torchwood had.
Everybody but the Doctor took turns taking care of baby Donna. Even Jack did a few diaper and feeding duties. As for the Doctor, this first version of him seemed even more cantankerous than the version Jack had met all those years ago in WWII. His appearance definitely confirmed Jack's theory of the Doctor looking younger as he regenerated. To make matters even more difficult, he had no memory of his later selves. Kept demanding to be released from his captivity so he return to Gallifrey to see his new granddaughter. That tore at Jack's heart.
So when he was wrapping up things with some Weevils he and Ianto had caught that night, he smiled at the sight he caught. The Doctor was cradling Donna in his arms, singing softly to her at what could have only have been some ancient Gallifreyan lullaby. He quietly slipped out of the room, unnoticed by the Doctor.
When Jack next caught the pair next in the TARDIS immediately after they had changed back to their correct ages, incarnations, and appearances; with no memory of the last two weeks, Jack decided not to mention what had happened to the pair.
Some things were better left to the past.
* * *
When they woke up a couple of Earth weeks later in the TARDIS embraced in a hug, neither one could quite remember what had happened. Nor how they came to be in the TARDIS with him in his first self's clothes holding Donna as if she were a child. Jack declined to offer up more than a few sparse details of what had happened to them, nor what the device was. Only, Jack had said that it was "Sealed up in the most secure vault available."
Given all the possible scenarios, the Doctor decided that he could accept those few answers.