My absolute favorite Doctor Who serial ever.
It has it all. Unseen alien race, paranoia and deception, thrilling cliffhangers, another great guest cast - especially Ronald Allen, and Action by HAVOC!
This story also introduces the "sting" heard at the cliffhanger leading into the end credits - a device still used to this day!
THIS is the story that solidified me as an obsessive Doctor Who fan and cememted Jon Pertwee as MY Doctor.
(On a side note, I love Cyril Shaps in this one. He plays "nervous" and "twitchy" very well in several Doctor Who stories, but in this one he is especially nervous and twitchy!)
SeaDevil