NN5 - The Abominable Snowmen 5

The war between science and religion that often takes place on Doctor Who results in a humiliating defeat for religion and Buddhist philosophy with the death of Rinchen. When confronted by a Yeti within the monastery, he prays to the "she-devil" (Victoria) for her to banish the beast. Instead, the Yeti kills the monk by toppling a massive statue of the Buddha on him. Symbolism has never been so cruel.

There's a couple genuinely unnerving moments in this episode. One of them features a quirky little performance from Deborah Watling when she is hypnotized by Padmasambhava, and all she can say is a panicky few words whenever The Doctor is in the vicinity. Another is the first meeting between Padmasambhava and The Doctor in 300 years, and the surprised reactions of both characters to the fact that both of them are still around to remember their first encounter.

We've heard The Doctor reference past, unseen adventures before, but this is the first time where one of these previous events have directly affected an onscreen adventure. At this stage in the show's history, it's just another layer in the mystery that is the character.