Turn Left is generally a cynical episode about the breakdown of civilization. Without the Doctor, crisis after crisis tore apart the fabric of society on Earth. Everything broke down, until the stars going out was practically an anticlimax. And the body count was freaking insane: if you appeared in the next episode, you died in this one. But the funny thing is, while the episode is very cynical about civilization, its very optimistic about people. Donna and her family bond with a man theyd never have met in a normal life, and end up singing drinking songs in the midst of a slow apocalypse. And Rose crosses between universes again and again to find Donna. Its revealed in the last act that, even without the Doctor, there are people who have been working to fix the world. Theyve found a way to adapt time travel technology, and theyve come up with a plan to fix the universe. The price is Donnas life, and she pays it. She just sends up a prayer to anyone listening: please. An ordinary person who, time and again, becomes extraordinary. Donna Noble.