As this episode is sadly missing, I base my review on the animation and radio recording, both of which I've enjoyed many times.
I'm often split when I consider Troughton's best Cybermen story, I'm split between this and Tomb, Tomb is very much an adventure story, this has way more of an espionage/thriller vibe about it, typical of the 60's and 70's, we're treated to lots of cloak and dagger, it's all very sinister, the music adds to that feeling wonderfully.
It's such a refreshing change after The Mind Robber, that was very surreal, this is totally the other end of the scale. We are treated to an episode of intrigue, Troughton is very much at his best, he is outstanding here. Kevin Stoney, even after a few seconds, provides an excellent villain with Vaughn. Great how nothing was given away.
Great start. 9/10