At our most recent Doctor Who Viewing Party, we watched Human Nature/Family of Blood. I love this episode. The premise of the episode is that the Doctor and Martha are fleeing from an alien species called The Family of Blood. This Family has a life-span of three months, and in order to survive, they must consume someone. This time, they have their eyes set on a Time Lord, with immortality as their goal. They are able to "sniff" out the Doctor, and in order to hide, the Doctor uses a chameleon device to re-write his biology to become a human, with his Time Lord-self contained in a fog watch. Martha goes undercover as the Doctor's maid in a pre-WWI British boarding school.
Throughout the course of the episode, the Family gets closer and closer to finding the Doctor, until finally Martha has to go through with her instructions to open the fog watch if the Family did indeed find them. In a nutshell, there are various complications, and the Doctor, who was going by John Smith, must make the decision to turn himself in. He goes aboard the alien ship and uses what he calls "ventriloquism of the nose" to trick the Family into thinking he was still human. He then causes their ship to explode, and we see the Fury of this Time Lord revealed. Just watch the video. That'll explain in better than I can.
I really liked this episode. It was a great combination of hilarious, serious, a little romance, and just straight up awesomeness. I loved how it showed the Doctor as someone other than himself, someone fallible, some part of him deep down inside that didn't normally show itself. I would give this two-parter as a whole 3.5/5 stars, but 4/5 stars for the ending of Family of Blood.