This has to be a contender for one of the best episodes of this year.
Tom MacRae has crafted a truly emotional romp that features the relationship between Amy and Rory better than any other episode. I thought Amy's Choice was their definitive story but no, its this one. With really only the three regulars as characters in the story it gives it room to breathe and fully immerse itself in their relationship to its fullest.
I really liked MacRae after Rise of the Cybermen but for some reason, that story has not proven popular with fandom. (Probably because the Cybermen in it aren't the originals, but that seems petty to me.) I think that story was underrated and hopefully this episode has redeemed him in the eyes of his critics.
Performances from both Karen Gillan and Arthur Darvill have never been stronger. Gillan truly manages to capture her character at two different stages of her life with great ability. And Darvill does a fantastic job showing both love and compassion towards Amy and resentment towards the Doctor.
Speaking of which, I can't help but feel this is the beginning of the end for the Ponds as companions. After the repeated dangers the Doctor has put both Amy and Rory in such as not bothering to check if their ever was a plague on the planet as Rory points out; and the clear resentment and horror Rory shows when he realizes the Doctor is making Rory more like him by making him choose between the two Amys, I can't see him wanting to stick around much longer. Oh, and them loosing their daughter to the Doctor's enemies of course.
Which brings up one the few flaws of the episode; the lack of a mention of River/Melody. It really stretches credibility that in such an emotional episode about the couple that there would be no mention of their daughter at all no matter how much they realize she's stuck in the machinations of the flow of time.
There are some other logistical problems with the episode like the nature of the disease conveniently not affecting Amy and Rory even as carriers. Why a resort planet wouldn't program its Handbots to recognize aliens. The unlikely scenario that they would be able to set up 10,000 different time streams for people and the immense energy that would probably take are things really best not thought about.
The story was about Amy and Rory and in that it accomplished its goals with flying colors.
Here's the list of nineteenth stories for each doctor from most to least favorite:
1. Human Nature (The Family of Blood) by Paul Cornell
2. The Girl Who Waited by Tom MacRae
3. The Green Death by Robert Sloman and Barry Letts
4. Planet of Fire by Peter Grimwade
5. The Seeds of Death by Brian Hayles and Terrance Dicks
6. Mission to the Unknown by Terry Nation
7. The Invisible Enemy by Bob Baker and Dave Martin