and the tv summary description makes me waver and have doubts about viewing it. here's why:
the TARDIS crashes in 1930, and the Doctor meets a war criminal. That's the text onscreen.
Now, Moffat may do a lot of things, but he wouldn't describe Churchill as a war criminal - even with the bloodbath on the beaches of Gallipolli.
The episode title gives us a clue: Let's Kill Hitler.
Now, is the Doctor going to hold Hitler accountable for the death of 6 million people? Recall that the Doctor doesn't like others holding the Doctor accountable for events the Doctor hasn't done yet. That doesn't wash either man's hands clean...it just demonstrates the need to be sure the right charges are leveled in any declaration of war criminality.
So what happens to Hitler? Does the Doctor do away with him? (give the Daleks a pet?)
On one hand, the Doctor doesn't have any problem with changing history, so long as the change is a result of someone standing in the Doctor's way (remember the Christmas Special that introduced us to Torchwood? the Doctor destroyed the career of the woman who was supposed to herald the start of a glorious period of human history)
On the other hand, the Doctor doesn't even help his friends much - among other examples, if someone helps you defeat the Gelf, wouldn't you at least tell him he's dying? that's if you weren't going to whip out your sonic screwdriver and give him another decade of life.