In the Tenth Doctor swansong story The End of Time, Part 2, the entire population of the Earth (sans Donna Noble, the Doctor, and a couple of spiky aliens) are transformed into copies of the Master. And not a single human character has mentioned it since. Viewers might be forgiven for thinking that such a profound, widespread transformation would be the cause of some longstanding consternation and trauma among the human population. In the real world, of course, such an event would be psychologically devastating, but this is the Doctor Who universe and so these events must be wiped from the ongoing narrative because, once again, they are inconvenient to all subsequent stories.