10 Dark Doctor Who Facts That Are Never Spoken About

Gridlock is the ultimate advert for walking. It features a giant motorway that keeps its passengers trapped in their cars for decades, with giant crab monsters lurking at the bottom that crush any vehicles that get too close.

The motorway was actually used to keep the inhabitants of New New York safe from Bliss, a mutated "mood drug" that killed the vast majority of the population.

Mood drugs are set up earlier in the episode, when a young woman buys one called "Forget" in order to erase her memories of her family, who went on to the motorway and never came back.

There's just one problem with that – they do come back.

The Tenth Doctor opens the motorway at the end of the episode, freeing those trapped inside and allowing them to return to their old lives. But what about their relatives not on the motorway, who were so consumed with grief that they bought a drug to forget them? What happens when the woman from the start of the episode is reunited with her parents, but has no idea who they are?

It's going to make for a complicated Sunday dinner, that's for sure.