RR2 - Fury From the Deep 2

The above picture is from one of the most unsettling scenes in Doctor Who history, which occurs during this episode. Thankfully, the Australian Censor Board thought the scene was too frightening to include for broadcast, too, and so excised it, left the trim in a box somewhere for 30 years so it could be found and enjoyed once more by everyone. Pity they didn't just trim the entire episode, then we could have watched that, too.

Mr. Oak and Mr. Quill (the latter of whom is pictured) are two genuinely creepy Doctor Who characters. They're not villains and they're not creatures. They're just two seemingly harmless maintenance workers on the base, but from the first time we see them, it's clear that they're not...right. Mr. Oak speaks in a voice that's just on the threatening side of gentle. Mr. Quill doesn't speak at all. When they coerce their way into Mrs. Harris's room and knock her out with some sort of gas, it looks and sounds (with the constant heartbeat sound effect of this story used to its full effect) scarier than anything any legion of Daleks, Cybermen, or Yeti could ever muster.

And all we've seen in the way of "monsters" thus far is a couple shots of some limp seaweed. As Van Luytens suspects at the end of the episode, whatever is threatening everyone in the refinery is "Down there...in the darkness...in the pipeline...waiting." This is frightening Doctor Who at its very best.