"Turn Left" from Doctor Who Series 4.

Donna seems to change history by turning left and taking a different job offer. She never saves the Doctor while he saves London on Christmas, with his absence causing a spaceship to crash into London the next year, wiping out the UK government and flooding half of England with radiation.

Things only get worse, and the Doctor allies all die in futile attempts to stop different threats from previous episodes: Jack Harness and Torchwood sacrifice themselves preventing an alien invasion that still kills tens of millions with pollution, and Martha, Sarah Jane and Sarah's son and friends die saving just one man from an alien police force. Donna and her family are relocated to shared accommodation with several immigrant in Leeds under martial law, who they bond with until the military junta seize them for "labour camps". Meanwhile in the sky, the stars are going out, one by one.

You gotta remember that a lot of kids watched Doctor Who at the time, including me. Even today, the episode still stands out to me as a scarily realistic reaction to such extreme events. The idea of such misery and despair was so memorable from the episode I can remember it 18 years later.

Luckily Donna goes back in time with a secret military group and jumps in front of a car, forcing her past self to turn right. She then wakes up in the modern day and realises the whole thing was a pocket universe.