There's a whole lotta mistaken identity here with most of the story having the Doctor and friends being accused of various things such as killing the anthropologists friend. It does get cleared up but not before the Doctor spends awhile cooling his heels in their equivalent of the hoosegow. I mean, how many times is the Doctor going to be arrested anyway? Seems that every third story he's being arrested over something or other.
The space aliens are not revealed until late in the story. What we first encounter are Yeti, then we encounter metal spheres, and then we learn the spheres are some kind of activation units that turn on the Yeti, and that the Yeti are in actuality robotic. Hurm.. these aren't the real Yeti but some kind of fake. There is obviously someone controlling the Yeti Robots, right? We eventually learn that the spiritual master of the monastery, said to be Padmasambhava (but can't be the real Padmasambhava), had encountered this entity while on the Astral planes and that this entity was using Padmasambhava's body to conduct an "experiment". But the experiment with these robotic Yeti has gone awry.
The Great Intelligence is this entity, and it is able to hypnotically control people.
In a great battle the Doctor and some helpers are able to destroy pyramids of spheres and other control equipment that were used to control the Yeti. Unfortunately the monastery is also destroyed in the battle and the monks are told to flee.
One of the story's conceits is that the Doctor had previously visited its location, in an earlier untelevised adventure. Although a few earlier stories contain passing references to the Doctor's unseen exploits (for example, The Celestial Toymaker), this is the first story where the Doctor's previous (unseen) knowledge of the setting and its characters are crucial to the story's development.
The North Wales mountain pass at Nant Ffrancon doubled as Tibet for the filming of this serial.... okay, it wasn't in the Highlands.