4. Doctor Who - "Turn Left"
This imaginative episode is an exploration of how one seemingly insignificant action can change the entire world. Instead of making a right turn, driving to the job that would eventually lead her to the Doctor, Donna makes the decision to turn left. Having never met the Doctor, she doesn't get the opportunity to save him from the Giant Spider-shaped Pun Machine (I think her official title was Empress of the Racnoss). So the Doctor dies. For real this time. Without the Doctor, there's no one to defend the planet from the myriad alien invasions that seem to plague London every few weeks. Everyone in the hospital from Smith and Jones dies, except for one medical student. The Titanic crashes into Buckingham Palace, and the entire south of England has to be relocated to the North. Conditions deteriorate, and eventually people are being relocated and even sent to internment camps. In one particularly chilling moment, an Italian immigrant family is sent off to a "labour camp" as the result of xenophobic sentiment that always seems to spring up in times of national crisis. It's probably one of the darkest and most fully realized alternate universes on here, and one that makes it incredibly clear exactly how much the world owes the Doctor. And all because Donna made a left turn instead of a right. Thanks, Donna.