Written by Russell T. Davies

Directed by Alice Troughton

If you’re still dubious about my assertion that Doctor Who is essentially a horror series, I direct you to this episode, which would not feel at all out of place in The Twilight Zone.

Like many episodes of The Twilight Zone, this episode gives us an unknown outside threat that forces a group of strangers into a trapped, claustrophobic situation in which they turn against one another. It’s a dim view of human nature and the then-current political climate that often pointed Rod Serling toward these types of situations, which makes it a surprising similarity for Doctor Who, but nonetheless, then-current show runner Russell T. Davies crafted an episode about a mysterious apparent life-form that is able to take control of the people on a disabled tour bus full of humans and one Time Lord.

The setting is interesting–a planet whose atmosphere of high pressure and no breathable air is conducive to the creation of diamonds but impossible for known forms of life to survive, leaving it a tourist location due to its absolutely beautiful scenery but inability to sustain life. And it’s the perfect setting for a form of life so alien that it has no visible form and seems to have no similarity to the humanoid life we’re used to on this show. The atmosphere means that the life-form would have to be very different from life as we know it, it helps create the claustrophobia for the humans, and it only adds to the mystery that the never-identified creature provides when no one can look outside for it, for signs of its existence, for its home, or even go out to study it after the episode ends.

However, it’s the creature itself that makes the episode so great. It’s never identified. It’s never named. It’s never even really described. It appears to possess one woman, then proceeds to repeat everything said in its presence with increasing rapidity until it finally apparently attains the ability to speak first, leaving the Doctor trapped under its spell, with the busload of people completely unaware of exactly how dangerous the Doctor under someone else’s control could be.

The Doctor is rarely in the dark (and David Tennant’s Doctor is a confident, dashing sort in general), but even he has no idea what to make of this life-form, which makes it even more terrifying than his fellow passengers could know. And Donna isn’t there to act all sure of herself the way she usually does, so we don’t have anything to give us confidence. It’s a terrifying experience, the essence of horror.