To begin, this is definitely my favorite episode. It was amazing. And it had Colin Morgan, who plays Merlin in the BBC series, and I love him, so that was definitely part of it.
So the Doctor and Donna are on this planet, taking a vacation. Donna stays behind to stay at a spa and rejuvenate. The Doctor decides to go on a tour of the planet, to see the diamonds on the planet. This episode was just so wonderful. Everyone is very awkward on the shuttle at first, but then the Doctor (who is obviously an extrovert) starts talking to everyone, and they're all laughing and having a good time. And Colin Morgan is being awesome and Goth, and wonderful.
When the shuttle shuts down. It just stops working. The stewardess says that it's just part of a routine check, nothing to worry about, but the Doctor isn't fooled. He goes back, using his Psychic Paper and discovers that nothing happened. It just stopped. They open the front to look at the planet. It is gorgeous. The diamonds form large formations (which is redundant I suppose), and the Doctor and two captains/pilots just stare at the beauty. Then one of them sees a shadow. It moves. Then they close it again, and the Doctor goes back out and confirms what the stewardess had said. He guaranteed nothing was wrong. Right at that moment something crashed into the side of the shuttle. Famous last words. Eventually whatever it is outside (after some truly brilliant creepy stuff happening) gets inside, and goes inside one of the people on the shuttle. It repeats everything they say, then says it at the same time as them. They all want to throw her out of the shuttle, but the Doctor won't let them. The creature is making them all argue. Eventually the creature steals the Doctor's voice, and convinces everyone that the Doctor has the creature now...I'm not gonna spoil the ending. But I love this episode. Love it. It was so intense. This is also the episode where the darkness that kind of pervades the next six or seven episodes begins, as it steadily gets darker and darker and darker until the End of Time.