Doctor Who has always been equal parts smart and silly. This is a tonal blend that has often backfired, such as last season's finale. This, however, is an example of when it really works. It's an amazing setup, and I really hope it doesn't get wasted like the last finale.
For the first minute or so, it seems to be a typical Doctor Who episode; weird creatures (that look like the Pale Man in Pan's Labyrinth) attack a young boy, and the Doctor shows up to save him. Then, things took a huge turn that brought back the moral ambiguity I loved from last season in a very interesting way.
Then, the show rights one of the wrongs of last season's finale, bringing back Missy, and reciprocating her tonal clash. She provides Clara a chance to shine in a scene demonstrating what a great companion she is.
Now, the two of them reunite with the Doctor, and this is where the silly comes in. As hilarious as the Doctor's entrance is, I almost started to worry that it was going to go too far and ruin the wonderful thing they had going, but Moffat pulled back just in time.
The final act involves Daleks, and is very clever in the way it ties in to the old show. It all ends with a shocking conclusion/cliffhanger, that makes you wonder how much is going to stick in the long run, and how much is going to be conveniently fixed in the next episode. We can only hope that this won't be wasted like last season's penultimate was.