This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Sapphire and Steel
The basic concept of the show: the universe is constantly at risk from hostile temporal attacks, and the best any human involved can hope for is that the "nice" Humanoid Abominations don't decide that killing you is the best response.
The prison and eventual fate of Ruth and the landlord.
Even though Ruth's friend Liz survives, she will have to take precautions for the rest of her life, and if she slips up even once the monster will be able to get at her if it ever escapes from its can — even if it doesn't escape until long after she's dead.
Harsher in Hindsight - Good luck to her following Steel's advice with the increased prevalence of CCTV and ubiquitous consumer photography e.g. smartphones over the past 30 or so years.