The City and the City (2018)

The barrier here is a learned act of perception: residents of two overlapping cities must refuse to acknowledge the other city's people and streets. A murder forces David Morrissey's detective to navigate that rule and the authority enforcing it. For Silo fans, the reward is seeing a seemingly impossible social arrangement made ordinary through habit, fear and paperwork. The 2018 four-part adaptation is a compact, unusual choice if you liked the question of what a population can be trained not to notice.