Probably the best Base Under Siege of Troughton's era; claustrophobic and eerie with the very present way in which their opportunities are literally cut off one after the other with the interference of the Great Intelligence. I found this story to be much better developed than its predecessor, The Abominable Snowmen, possibly due to the longer runtime giving the narrative more room to stretch itself out. I also found the way the story played with our suspicions of various characters over the course of six episodes to be incredibly suspenseful (even though with the benefit of hindsight, we're at least certain that it can't be the Brigadier Col. Lethbridge-Stewart).