The TARDIS lands in 1935 Tibet.  The Doctor visits the Detsen monastery so that he can return a sacred bell.  They meet a man named Travers there who is looking for Yeti.  The Yeti turn out to be robot guards.  The high lama of the monstary Padmasambhava has been taken over by the Great Intelligence.  The Great Itelligence is a non-corporeal being that has prolonged Padmasambhava's life.  It uses him to control the Yeti.  The Great Intelligence's intends to create a form for itself from which it can take over the Earth.  The Doctor is able to overcome it, however, and save they day.