Here there be spoilers, so don't say I didn't warn you. Though I think most fans have already seen this 50th anniversary special by now. I haven't seen any of series seven, the 11th Doctor's final season, so I was a bit reluctant to watch this right away. Based on advice from wise council, who said this show could be watched without that seventh season viewing, I finally sat down and watched the whole thing. And it was a lot of fun, for many reasons, some of which I will enumerate in the space below.
The whole invasion plot with the Zygons was mostly a Macguffin, to get the rest of the story started. I'm not sure how I feel about there being a minus one Doctor, but for this story it works out just fine.