A very impressive cast, some great production values, and a decent story make for a very good episode.
The cast is impressive, the performances perhaps don't live up to the standard, Bob Hoskins is decent without having a huge amount to do, Susan Hampshire is good, even if her American accent is a little off. I don't understand why the need for a token American in every episode, particularly if they're British.
I don't mean to sound negative highlighting those faults, because this is a very good episode, I've mentioned the cast and strong production values, but the success of this one comes from the fact that there are multiple threat sources, you have no idea initially where the main threat is coming from, as plenty of characters seem to represent a threat.
Not vintage, but still very good. 7/10