That was one freaky episode. They are in this shuttle, with windows shut because of radiation, where they are going to a waterfall made of sapphire when this weird creature attacks. Basically, it took over the mind of a person, repeating what people said, creeping in inside people’s head, eventually ending in paranoia. It was quiet different from other episodes. It was kind of like a psycho thriller, with one of the very few times in which the Doctor is helpless. Anyways, a must see.
Doctor Who: midnight (spoilers)