Regardless of the business wranglings that led to the revamped Doctor Who's sixth series being split in half, the main effect it has had on the show itself is that it allowed show runner Steven Moffat to have two series finale-style cliffhangers for the price of one. Following the revelation overload that was A Good Man Goes to War, in which we learned that The Doctor is the hated enemy of a religious order, that that religion kidnapped Amy and Rory's baby Melody in order to raise her as a weapon to destroy The Doctor, and that Melody would grow up to be River Song, it seemed like the show might return with something a bit more frivolous, what with all the death and war and baby stealing that made up the last episode. When that episode ended with the legend "The Doctor will return in Let's Kill Hitler", that seemed like it would be exactly the case.
The title proved to be something of a bait and switch, though, since the historical romp that the title suggested turned out to be a pretty dense mythology episode. Rather than being an episode about The Doctor, Rory and Amy trying to kill Hitler - they in fact accidentally save his life, then bundle him into a cupboard, at which point he is never seen again - it was about the birth of River Song. Not the birth of Melody Pond, which we have already witnessed, but the creation of the woman whose life (and death) will be inextricably linked to that of The Doctor.
Though she doesn't start the episode as either Melody Pond or River Song, but as Mels, played by Nina Touissant-White. Mels is introduced when she drives a stolen car through the crop circle that Amy and Rory have carefully drawn to contact The Doctor, almost running the three of them over in the process. We then see a fun little montage showing how Mels is actually an until now unmentioned lifelong friend of Amy and Rory's. A inveterate troublemaker who knows all about The Doctor, starts off by saying that Amy never told her how hot he was, then pulls a gun on him, she forces everyone into the TARDIS and utters the line that gives the episode its title, "You've got a time machine. I've got a gun. Let's kill Hitler."
Once in Nazi Germany, they crash through the windows of Adolf Hitler's (Albert Welling) office, accidentally thwarting the efforts of a squad of miniaturised time travels riding inside a shape-shifting robot (yeah, it's kind of really dumb) to kill him. When Hitler starts shooting at the robot, Rory knocks him out, but not before an errant bullet hits Mels, which - Melody being part Time Lord and all that - causes her to regenerate into River, revealing the fun little fact that, since Amy and Rory named baby Melody after Mels, that means that they named their daughter after their daughter.
From there, River tries to shoot The Doctor, but is constantly prevented to do so thanks to a couple of cleverly staged switcheroos The Doctor secretly conducted, and which the show delights in going back and showing us, before kissing him on the lips, introducing a poison into his system that will kill in him thirty two minutes. (The series of revelations about The Doctor and River's game of one-upmanship reminded me of the fun 1999 Comic Relief special, Curse of the Fatal Death, which was written by Steven Moffat and got a lot of mileage out of The Doctor and The Master constantly time travelling in an attempt to kill each other through inventive architecture. It can be seen in two parts on YouTube.) River runs off into the streets of Berlin and causes havoc, Amy and Rory give chace, and The Doctor prepares to die by heading back into the TARDIS and changing into top hat and tails.
The story is primarily about changing River from the would-be assassin that she starts the episode as into the unshaped clay of the character that she will become, which it does by having her witness The Doctor's resilience, even in the face of imminent death, as he tries to save Amy and Rory once they are miniaturised and put inside of the justice squad's robot, which is patrolled by antibodies that kill anyone who doesn't have the correct privileges. In generally, I thought that the robot was really stupid, and all the stuff that happened inside of it was not very well handled, but I did really like the deadpan humour of having the antibodies be incredibly passive-aggressive in their dialogue, calmly saying things like, "Welcome, you will experience a tingling sensation, then death", and "Welcome, remain calm while your life is extracted." River gives up her ability to regenerate in order to save The Doctor, in doing so laying the groundwork for the person she will one day become.
I enjoyed Alex Kingston's work in this episode more than I have in a whole. Due to the nature of River's character, being someone who knows pretty much everything that will happen in The Doctor's life, she can at times come across as gratingly smug. She did this time around, but at least since she was ostensibly the villain of the episode, it made sense for her to be pretty obnoxious, and her growth over the course of the 45 minutes was very nicely played.
Outside of the River story, the episode threw out a couple of small but very important pieces of information; The Silents, in Buffy terms the Big Bad of this season, aren't a species, but a religious sect convinced that the universe will end when someone asks a specific question (which I guess makes them the apocalyptic equivalent of the people who built Deep Thought in The Hitchhiker's Guide To The Galaxy); the fact that Melody Pond grows up to almost kill The Doctor in 1938 suggests that maybe The Doctor doesn't get her back when she was a baby, or something else happens to her along the way; and The Doctor, given access to the files contained in the robot, now knows that he will die on April 22nd, 2011, as seen in The Impossible Astronaut, a fact that Amy and Rory have been trying very hard to keep from him because a little foreknowledge is a dangerous thing. Incidentally, hats off to Matt Smith, who was at his exuberant and rascally best here, even as he was acting out his second death of the year.
I'm from the school of TV science fiction fans who really like mythology episodes. I loved Lost and The X-Files, for all their myriad flaws, because every so often they would make episodes that were just about deepening the world of the show. For that reason, I really liked Let's Kill Hitler because it revealed some things about River, a character that I find interesting, and because it moved some of the pieces of the broader series narrative into place. But as a standalone episode, I didn't think it really worked. The robot was, as I've said, kind of a stupid idea, and it never felt like anything other than a device to advance the mythology elements. It was a mixture that didn't quite gel, which is a shame, because the mythology stuff was really fascinating.