Everything about this is wonderful. Mars and the Bowie Base are so wonderfully realised that everything feels just so real, which adds to the dread, claustrophobia, terror and unnerve. The crew, while not all given the same depth universally, are all given some insights to really root for them, with Adelaide Brook being such a fantastic character.

The Flood are terrifying, and such a wonderful concept, and building fear from such a mundane thing.

And it leads into the Time Lord Victorious, which is such an interesting concept, and adds depth to the Doctor.