After the last episode, the Doctor goes off and leaves the Ponds for a bit, so they get his attention by creating a crop circle. I love that it says “Doctor”…
Anyway, Eleven shows up and they catch up a tiny bit before hearing sirens in the background. Turns out Amy and Rory’s childhood friend, Mels, has stolen a car and needs to get out of there. See, Mels has kind of a bad streak, one that Amy has had to get her out of for years and years. Trouble at school, trouble with the law… She was also the one that pointed out that Rory liked Amy and pretty much pushed their relationship along.
Anyway, there they all are in the cornfield and Mels wants to go on an adventure with Amy’s raggedy Doctor, so they do something that every show involving time travel does at one point: tries to kill Hitler.
But before they show up in 1938 Berlin, we see this guy turn into another human! Well, it looks pretty computer-y in some way and we learn it’s a Teselecta, which is kind of a person-shaped ship with tiny people inside, steering it and whatnot. This Teselecta guy turns into the other guy right in front of his eyes! He’s some sort of right hand man to Hilter. Anyway, at the end the robot actually takes the glasses right off the guy before killing him. Then it’s off to kill the big man himself. Of course, that’s right when the TARDIS makes a crash landing right in the room (Mels fired a gun in the TARDIS, not a good idea…). This throws the Teselecta off enough to give Hilter time to get his gun out and shoot, but he misses the robot and gets Mels. They take care of her, of course, and lock Hitler in a closet (like you do), but then right in front of their faces, Mels regenerates! Turns out she’s a time lord, and she turns into River Song!
So, now we know that the little girl in the spacesuit regenerated into Mels, Mels grew up with Amy, who practically raised her, and then she turned into River.
Anyway, she starts trying to kill the Doctor – guns, knives, anything really. The people who kidnapped Melody have trained her to do just that. Then she gives him a kiss and, before running away on a vespa, tells the Doctor that it’s poison on her lips and that he’s going to die. Since the Teselecta goes back in history and brings justice to criminals and identify River as the Doctor’s killer, they go after her, and the Doctor (while dying) tells Amy and Rory to go after her too.
They chase her to a hotel and bring them on board the Teselecta with them, then it takes on Amy’s image to attack River. But before they can go through with it, the Doctor, still dying, shows up on the scene (a hologram of Amelia convinced him) in a top hat and suit, to stop the Teselecta’s crew. Turns out they belong to a religious sect that involves the Silence and believe that when “the oldest question in the universe” is asked, silence will fall across the universe. Sure. Anyway, the Doctor gets them all to “abandon ship” and leave Amy and Rory alone. Mels/River saves Amy and Rory, but it’s too late for the Doctor, who whispers something in her ear before dying. She asks Amy who this River is, and the Teselecta turns into River, aka Mels post-regeneration, and so she sacrifices her remaining regenerations to save the Doctor, then reasonably passes out.
When she wakes up, the Doctor gives her the famous TARDIS notebook as a present before they all leave her at a future hospital. All is well again – River’s studying to be an archaologist, she’s not trying to kill the Doctor, and everyone knows that River is Amy and Rory’s daughter. However, the Doctor has learned the date of his death at Lake Silencio, so there’s that… And this is the last time we see River for a while, so we’ll say a temporary goodbye to her now. Wonder if she’ll be back in the new episodes…