The Abominable Snowmen

From http://www.bbc.co.uk/doctorwho/classic/episodeguide:

The TARDIS arrives in Tibet in 1935 and the Doctor visits the remote Detsen monastery in order to return a sacred bell, the ghanta, given to him for safe keeping on a previous visit. There he meets an Englishman, Travers, on an expedition to track down the legendary Abominable Snowmen or Yeti.

It transpires that the Yeti roaming the area are actually disguised robots, which scare away or kill anyone who approaches. The High Lama Padmasambhava, whom the Doctor met hundreds of years earlier on his previous visit, has been taken over by a nebulous alien being, the Great Intelligence, which has artificially prolonged his life and is now using him to control the Yeti by way of models on a chessboard-like map.

The Intelligence's aim is to create a material form for itself and take over the Earth. The Doctor banishes it back to the astral plane, allowing Padmasambhava finally to die in peace.

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