4C2 - The Ark in Space 2

If you had any doubt about how the Philip Hinchcliffe era would be different from any era that preceded it it, look how Kenton Moore as Noah reacts at the end of this episode to the fact that his left hand is becoming something alien. The look of sheer horror on his face in reaction to what is little more than his hand caked in green bubble wrap is nowhere near what we would see in a story from Seasons 1-11, mostly because such a scenario had barely been addressed before.

That scenario? Alien possession of a human mind and body. It's such a frightening concept that is so far removed from the "Yeti sitting on your loo in Tooting Bec" situations that Jon Pertwee's Doctor had to deal with. Not only was the setting of this new Doctor Who different from what viewers were used to, but so, too, were the threats seen on screen. No longer would a viewer look at a household object and imagine it was an Auton in disguise. That viewer now had to cast his mind outward into the vast expanse of space and time. Doctor Who was now challenging the viewers to think beyond their normal boundaries.

While the stories of the 1960s journeyed into the great beyond often enough, the aspect of horror was seldom touched upon (only Fury From The Deep, which features alien possession, but is set at a gas refinery on Earth, comes close to the levels of horror in The Ark in Space). And, just so long as one can overlook the slight dodginess of the effects in this episode (and the story as a whole), there's plenty to be scared about here, too.