Kill The Moon took something that we know - the moon is an lifeless satellite orbiting the Earth - and turned it completely on its head. In true Doctor Who fashion, the moon was something else entirely; an egg containing a giant jellyfish-like alien creature. The spiders we saw in the trailer weren't aliens, though they did pose a deadly threat to the Doctor and his companions. They were bacteria, growing under the surface of the moon and like a nightmarish Demestos advert, were breaking out and killing everything in their path. Rather cleverly, the mystery of how the moon was still orbiting the Earth in the future was solved; the alien within laid another 'moon egg' in its place. Though you have to wonder whether it escaped the destruction of Earth in series one's The End Of The World.