Well, this was fun. My one and only gripe about this was that it was so short!
Yes, I know, these stories all are, and they're supposed to be, but... I'd have loved
for this particular one to be longer. Especially as it felt a little bit unbalanced, as if setting up the story (as well as explaining this incarnation of the Doctor and his circumstances to new/young readers not familiar with him) took up considerably more time - and was actually more detailed and more satisfying, in a way - than the ending/resolution, which all happened a bit too suddenly.
But that aside, I loved it. The Third Doctor was recognisably himself, Jo was adorable, the Master's appearance, while far too brief, was appreciated, and there were some really lovely moments with the Doctor and Jo.