This episode starts with Amy and Rory driving around like crazy to make crop circles, because apparently the idea of slowing down didn't occur to them, because they are retarded. Also, their lifelong friend "Mels" shows up. You know, Mels. She's been their friend since primary school. It's just a crazy coincidence that she's never been seen or mentioned before.
I should point out before I go on that I actually kind of liked Mels. The flashback stuff was awful, and the fact that they retconned her in (and then lampshaded it) was shitty, but she had potential. Which is why it was so crushing when she turned into River Song within about five minutes.
In case you didn't watch the episode, she literally turns into River Song. Because of the whole "River is a magical Time Lord/TARDIS/human hybrid" bullshit, she can regenerate, and Mels is actually her the whole time, but a previous incarnation. And then Hitler shoots her and Mels is gone and River is back.
And yes, that is the actual Adolf Hitler, most of this episode is set in Berlin, 1938, where a bunch of time-cops have come to arrest Hitler for war crimes, before realising that they're too early and he hasn't committed them yet, and they don't want to change history. So what's the point of arresting him if you don't want to change history? Punishment, of course. They plan to wait till the point where he was supposed to die and then capture and torture him.
Strangely, they don't seem to be intended to be regarded as the bad guys in this situation. I guess this would be less unexpected (but still would make them bad guys in my view) if it were just Hitler, but they also plan to do the same to River. Now, personally, I'd be happy to see River tortured, but I get the impression we're supposed to like her, so this strikes me as odd.
Then River attempts to kill the Doctor with an incurable poison, which she administers by kissing him. How is she not dying? She has poison on her lips! There's no antidote! It kills within about half an hour! She should be dead!
The Doctor, "dying", goes into the TARDIS and talks to a hologram of Amy as a little girl for a while, apparently to fill up a bit of time in the episode. There didn't seem to be any other point to this scene.
Meanwhile, Amy and Rory get taken onboard the torture-squad's Terminator, which they control from inside by being miniaturised. (And despite the obvious parallels and the fact that motorbikes are stolen, no one says "I need your clothes, your boots and your motorcycle. Give them to me." which I feel is a terrible missed opportunity).
So, in miniaturised form, they run around inside the robot for a while and end up in the control room, where, after a whole bunch of dicking around, they get the torture-squad to leave and then River somehow miniturises and pilots the TARDIS in there to pick them up.
Then River uses up all her remaining
If there were any attempt at keeping the continuity straight in this show, this would be the last we ever see of River Song. She and the Doctor supposedly meet in reverse-chronological order, so his last meeting with her is her first meeting with him. Well, this is her first meeting with him, so she should be gone forever now.
Unfortunately, the writers are a pack of braindead arseholes, and they think River is fucking great, so they're not going to let her go. She's going to be back again and again and again forever.
In the end, this episode wasn't as bad as it looked like it was going to be. That said, it looked like it was going to be worse than genocide, so that's not saying much. But even River wasn't especially annoying this time, just annoying like a mosquito flying around near your head when you're trying to sleep, a sort of whining hum that you just can't ignore, but if you turn on the lights to try to find and kill it, you'll never see it.
I guess not enough really happened in this episode for it to be too bad, it just blurs into the other terrible episodes with nothing much to distinguish it.