Well that had to have been one of the more imaginative Dr. Who episodes ever. The episode is called Blink, hence title of this post.
It starts out with an inquisitive young lass named Sally Sparrow who stumbles across a long abandoned house. Imagine her surprise when, after snapping a picture in a room, she peels off some of the wallpaper to eventually reveal 'Beware the weeping angel Sally Sparrow. And Duck. Really Duck. Now Doctor Who 1969.' And then she ducks as a rock goes whizzing over her head to strike the wall. And out in the overgrown garden is seen a statue of a weeping angel who's hands are covering her whole face.
These angels, we learn as the story progresses, are in fact assassins created in the far past who are virtually unbeatable. As long as a living person observes such a creature, the creature freezes into stone. But beware if someone takes their eyes off the angel or even blinks, for like when a boojum strikes, they will find themselves thrown into the past to die while the assassin feeds off the temporal energy.
Unfortunately for Doctor Who and Martha, the angels struck and sent them into the past. So using his wits, a bit of future tense knowledge of Sally Sparrow, and some luck; the Doctor rescues himself and Martha from 1969 while saving Sally Sparrow and defeating the weeping angels before they could pinch his TARDIS and destroy two-thirds of the universe.
I may not be able to look at another statue in the same light ever again as the ending plays on nicely. As I said, this was a bloody good episode.