Often a Classic Doctor Who story can make a bit of a hash of moving the characters around to where it needs them to be at a given time, but this story does so flawlessly. It's well-written, fun, light and uses the Yetis and The Great Intelligence better than The Abominable Snowmen.
A problem I get with Classic stories once I watch too many of them is the exact problem The Doctor laments in The Pirate Planet, that wherever they go, no matter what, they immediately get put in prison as the sole suspects of the trouble going on. This story toys with the idea of The Doctor and companions being suspicious, in a - I find - very natural way, without ever storing the gang away in a cell for 2 parts 'cause that's the only way the story can actually work. No, the suspicion levied at The Doctor and co. is used throughout the story to create a prevailing sense of paranoia, without anyone ever feeling paranoid, naturally created by being met by an enemy of unknown origin and motive.
The episode 3 twist set-up works really well, just before it happened I thought about the possibility of it happening and how his previous actions could make sense as a deliberately bumbling distraction from the real danger. Sadly it's turns out to be a fake-out to show how much of a self-serving sleazeball the reporter is, which is a bit disappointing. But, I guess necessary in a mystery, red herrings and all that, however, as red herrings go, I don't thing it's that good as the actions don't really make sense once you know he isn't being controlled by the Intelligence.
Notably this is the story known for introducing Nicolas Courtney as The Brigadier, though this wasn't Nic first story (that's Mission To the Unknown) and he's not a Brigadier, which I just think is fun.

All-together it is a fun little story, that decently written with a good sense of paranoia throughout, but it just lacks that special something for me to give it more than a...

7.5/10