Traveling to the vacation planet of Apalapucia (The second-most popular vacation spot in the universe), The Doctor, Amy and Rory quickly find that the entire planet is under quarantine. The good news is that the plague only affects beings with two hearts, so Amy and Rory are quite safe. The bad news is that Amy gets locked on the wrong side of the quarantine zone, trapped in an accelerated time bubble where her whole life will pass in one day - a kindness contrived by the natives of Apalapucia, so says The Doctor, because it allows the infected to live out their whole lives before the plague takes them while allowing their loved ones a chance to "watch them live instead of watching them die."
Kindness though it may be, Rory and The Doctor aren't crazy about that option, so they contrive of a way to use The TARDIS to break the time bubble and rescue Amy. They are successful, but arrive a relative 36 years too late. And the 60+ Amy Pond they find is not very happy with them and in little mood to cooperate in helping them save "the real Amy". Now, Rory is faced with a sadistic choice - which of the women he loves does he save?
* Surprisingly, Doctor Who has done very few stories about alien diseases and just what problems a space/time traveler might have in seeking medical treatment or what role they might play in further spreading a plague. It's a nice touch to see this sort of thing being addressed on the level of high-science, with the story exploring how one race might used advanced technology to try and solve a very human problem - coping with death.
* Arthur Darvill shines in this episode, doing an amazing job playing up the Rory's internal conflict. On the one hand, he feels guilty about what happened to Old Amy and feels he has a responsibility to save her after she waited for him to save her. There is also the unspoken suggestion that maybe - just maybe - he likes the idea of an Amy who fully understands and appreciates everything he did while waiting for her as The Last Centurion. But on the other hand, he still has a chance to save "his Amy". The Amy he can grow old with. The Amy who may not understand him but whom he can spare the pain of loneliness that he endured for her.
* Karen Gillian gets some of the best material she's ever been given as an actress in this episode and runs with it. We've seen time and time again what Rory would do for Amy but this is the first episode that truly gives us an idea of just how much Amy is willing to sacrifice because of her love of Rory. She was willing to spend years trapped in a pocket dimension, waiting on a rescue she wasn't sure was coming... because she thought he was worth it. She was willing to give up her life to give him a chance to grow old with her... a version of her, anyway.
*The theme of The Doctor staring into the abyss too long and now being just as much of a monster as the creatures he fights returns but never has it been lain out so plainly. Steven Moffat explored this concept recently in A Good Man Goes To War and in Series 5 in The Pandorica Opens. It even came up a few times during Russel T. Davies' tenure as showrunner, in episodes such as The Runaway Bride and Journey's End. Indeed, Rory echos the accusation Davros made to The Tenth Doctor - "You’re turning me into you!" This further links this episode to yet another reoccurring Doctor Who theme - the monster that turns you into a monster.
* While the make-up used to make Karen Gillian look older is quite good, one wonders why the BBC couldn't have sprung for a wig or some dye to give her a few grey hairs?
* One small logic problem: Amy has access to an Interface that can give her nearly any bit of information she needs. Why doesn't the Interface allow her to contact the people running the hospital and tell them that she doesn't belong there?
THE FINAL VERDICT
Possibly the best episode of the Series so far. Certainly the most touching.