With all the hand-wringing about whether "Let's Kill Hitler" was going to make too light of Hitler or not, I doubt anyone expected that he would be locked into a cupboard for the entire episode (note to Moffat--please continue with Roronicus Pond, action star).
I definitely did not expect an entire episode about our River Song, least of all that we'd get to see her first meeting with the Doctor so soon. So, while I literally shouted in surprise at least 5 times throughout the episode, it's now time to think more logically through the episode's implications (insofar as one can think logically through any product of Steven Moffat's brain).
THE MANY LIVES OF RIVER SONG
How many regenerations has River had?
Mels/River showed no surprise or discomfort about the regeneration process...so how many times has it happened? We know it happened once with the girl in the astronaut's uniform, but that was back in the 1960s. So she had to have regenerated at least one more time in between, to have been a child in the 1980s. Or does she? We do learn, after all, that River can program herself to age in reverse if she wants.
I propose this timeline of sorts:
1st Melody: spans from birth of Melody Pond to regeneration on the streets of New York City. It's during this time that Madame Kovarian (or whoever) train Melody into becoming a weapon to kill the Doctor.
2nd Melody: I'd like to think she regenerates back into a baby after the incident in NY, and Amy and Rory actually get to raise her this time.
3rd Melody: For Melody to demonstrate such knowledge and control over the regeneration process when she regenerates into Alex Kingston, it stands to reason that sometime in between Melody 2 and "Mels," she regenerates into an adult form and runs off and has adventures.
4th Melody: Perhaps Madame Kovarian or the Silence catch up to her, knowing that Amy and Rory are the keys to finding the Doctor, and thusly they plant "Mels" into their Leadworth lives.
So, the penny's in the air...and the penny drops:
5th Melody aka River: Dare I say the greatest Melody of them all. We don't yet know why she changes her name to River (but this could just be one of those ontological paradoxes, where the she learns that she will become River Song, therefore she becomes River Song. Now repeat after me: WIBBLEY WOBBLEY TIMEY WIMEY. That's better.).
Really wacky but totally true observation: I believe that after she's let out of prison, River is under house arrest on Earth for a time, much like the Third Doctor, and forced to work for the government. During this time, she becomes a public defense attorney. If you do not believe me, allow me to remind you that Alex Kingston had a recurring role on Law and Order: SVU. The name of her character? MIRANDA POND!! Moving on now...
If we take "Let's Kill Hitler" on its face, then we are left to believe that River has used the remainder of her regenerations on saving the Doctor. However, I think the construction of the dialogue suggests a Moffatrick:
Amy: "Apparently you used all your remaining regenerations in one go. You shouldn't have done that..."
River: "He said no one could save him. But he must have known I could."
Doctor: "Rule 1: The Doctor Lies."
For the Doctor to have said that at that particular moment means one of two things: either River did not actually use all her remaining regenerations in one go, or River did not actually save him. It's also possible that both are true.
ETA: Re-reading this post, I found that I ignored one super-glaring contradiction. Mels/River explicitly says this is the first time she's meeting the Doctor, but surely the first time she meets him is as Melody 1, when she kills him?
And let's not forget one essential part of River's parentage: the Tardis. Perhaps if the Doctor returns to the Library, he can take River's essence out and combine her with Tardis energy and make her real again (ok, now I really am just guessing).
But let's be honest. A Doctor Who world without River Song in it scarcely bears thinking about.
OTHER RANDOM NOTES:
-One rarely thinks about directorial flourishes in Doctor Who, but this episode had a few neat references to classic film. The best one, of course, being "Hello, Benjamin." Though, admittedly, it's odd that the Doctor doesn't get the reference in this episode when he actually introduces River as Mrs. Robinson to Nixon...
-Flipping River's gun for a banana hearkens back to two Steven Moffat works: "The Empty Child" and "Curse of Fatal Death". The manner in which the Doctor kept faking out River and the gun was almost identical to how the Doctor keeps foiling the Master's dastardly plans and replacing them with a "sofa of reasonable comfort."
-Is it just me, or did the Tesselecta make no narrative sense whatsoever? But as with most Moffat things that make no sense, that usually means they'll be coming back later. But they've definitely been watching too much Star Trek.
-A lot of people seem pissed off that Amy and Rory don't act more parental about Mels/River, but I thought it was beautifully underplayed. Karen Gillan's delivery of "Why are you a psychopath? You're not a psychopath" was one of the most beautiful moments I've seen on the tv, a genuine moment of angst that doesn't overwhelm the imminent danger they face.
-River easily wins the best line of the episode: “Well, I was on my way to this gay gypsy bar mitzvah for the disabled when I suddenly thought, ‘Gosh, the Third Reich’s a bit rubbish. I think I’ll kill the fuhrer.’”
In the comments, James linked to an interesting post which offers a potential explanation to the Doctor's sudden wardrobe change. I highly recommend it.
Anyhow, so long and thanks for stopping by! As always, feel free to hash things out in the comments. Tell me YOUR favorite line of the episode, etc.