The weeping angels! I actually just went on a poll this morning and voted the Weeping Angels as the creepiest Doctor Who villain/monster. They were winning by like 60% or something, but I digress.
The Doctor and Amy save River from a ship and join her on an adventure underground. They've found a weeping angel in a crashed ship--the very ship River was in, actually. They have a monitoring system that recorded about four seconds of video of the Weeping Angel. They're trying to figure out what to do, so they resort to River's TARDIS notebook. They left Amy to watch the video, when they discover that anything that bears the image of an angel becomes an angel. They discover this too lagte, because the Angel is advancing on Amy, who is now locked in the trailer. She's also born the image of the Angel. But she manages to find and use the blip on the tape to get rid of the angel so it doesn't kill her, and they get her out. They enter the ruins and quickly discover that all the statues are weeping angels. So the Doctor activates the crashed ship's antigravity and they all go into the ship. They eventually end up in a forest and Amy must keep her eyes closed or she'll turn into an angel. When the crack appears!
These episodes are full of wonderfully fun weeping angels frights (again, creepiest creature in Doctor Who), especially when they used the voice of that one dude and were just casually talking about their plans and the people they've killed...it was beautiful. I love Steven Moffat. I dunno if he wrote these episodes, but even if he didn't, he's wonderful. Anyways...there was also a lot of questions raised about River's connection to the Doctor, like all of her terms of endearment. The question was raised of her possibly being the Doctor's wife, and I really, really, really, just don't want her to be. I like her okay, but I don't like her enough to be his wife. You know? Like, if he was going to marry anybody, it should have been Rose. Just sayin'... but whatevs. I'll find out with the finale of season 6, which I'm so close to. So...the whole crack thing was also interesting.
And the ending...Amy has "decided" what she wants, but the Doctor knows it's just an illusion, which leads into the next episode, where...