- The idea of getting brutally murdered and getting replaced by an alien copy without anybody knowing is a bit scary.
- Potentially taken further with Scarlet; it's never analysed in depth, but according to all evidence, technically the 'original' Scarlet died in the pilot episode, so the one the series focuses on is 'just' a duplicate with all of his memories who nevertheless isn't actually him...
- The whole "Attack on Cloudbase" Episode if you discount the ending.
- Any scene with Captain Black... good God... the guy's Nightmare Fuel incarnate.
- How about the artwork in the end credits? Seeing the hero about to be crushed by closing spiked walls, tied up and sinking while sharks close in for the kill...
- Better still because we're used to these as James Bond death traps where the hero somehow escapes. Here, the full implication is that Captain Scarlet doesn't escape. He dies, painfully, and respawns.
- It's surprising that the kids of the 60's who watched this weren't scarred for life at the death toll. At least 6 people die per episode — people EXPLODE, fall out of airplanes, fall off dams, get crushed by car lifts, get run over, get shot in the FACE, and turn into living bombs.
- The opening credits put the camera in the viewpoint of someone sneaking down a dark alley who is suddenly caught by searchlights, unloads an entire clip into his attacker and is shot dead. By the Hero, who doesn't even flinch as he does so.
- In almost every episode, at least one person is killed so they can be replaced by a Mysteron "clone". Crushed in garage car-lifts (while cheery music (turned up loud, no less) plays from the garage radio), strangled with robot hands, brake lines cut. And then we see the dead body duplicated by the Mysterons. For true nightmare fuel, watch the end-credit sequences, where Captain Scarlet is subjected to ten different death-traps.
- It gets worse when Fridge Horror kicks in- he still feels pain. Not only that, but he may never die, meaning he's doomed to watch everyone he ever cared about wither away, until he is alone.
- The Mysterons themselves are Nothing Is Scarier incarnate. All that is ever shown of them are the two infamous green rings and the iconic booming Hell Is That Noise voice. They'd be unsettling even if they weren't trying to Kill and Replace humanity.
- "THIS IS THE VOICE OF THE MYSTERONS. WE KNOW THAT YOU CAN HEAR US, EARTH MEN."
Nightmare Fuel / Captain Scarlet and the Mysterons