Doctor Who S4: E11--Turn Left
Donna gets persuaded to enter a fortune teller's tent, where she's forced to change her past, and therefore the future. She never meets Lance, so never meets the Doctor, so never helps with various things there. And because Donna wasn't there, there was no one to stop the Doctor when he was killing the Raknoss, so he died, and so didn't stop a number of things he did stop with Rose and Martha. So the world falls apart, and the Holocaust starts again, but not just with Jews. Then Rose comes back. She tells Donna that the stars are going out, that the darkness is coming. That Donna changed the past, but needed to go back and turn left, not right, to set the world okay again and get the Doctor back, to mend the future. So they do. Simple as that. But this was a very dark episode, too, because the world was falling apart, and there was a very moving part when the lively foreign people living with Donna were sent off to labor camps. That's what they were called last time.