If I had to name the one weak link in the story so far, it would be Margaret. The writing and acting surrounding her and her romance with Peter have been adequate, but a little one- dimensional. Until now, her character has not measured up with the standard for subtlety and depth set by the other characters in my opinion. But in this episode, she gets a chance to shine. Elizabeth is out of the country on the commonwealth tour, meaning Margaret is left in charge. She immediately stirs up quite a lot of trouble when she tries to liven things up a bit, forcing her administration to intervene. It's fascinating to watch Margaret in this new position, suddenly interacting with characters whom the viewer is familiar with but she is not. Yet the episode does not just develop Margaret alone; it also takes leaps and bounds in revealing the relationship between her and her sister. The episode's final scene stands out as being particularly magnificent in this regard.
As a proud citizen of Canada, a (former?) member of the Commonwealth, it is fascinating to see the Commonwealth tour depicted in the episode. Not only do we get to see a historical recreation of the tour, but we also get a peek behind the scenes, into the inner workings of it.
One subplot in this episode that I think will be commonly overlooked is the Queen Mother buying a house in Scotland. It's a touching little story which highlights her character very well, though it is ultimately completely irrelevant to the episode's main plot.