The money-saving trick in this one was making aliens out of Woolworth's Beatle wigs and Ping Pong balls. I did wonder if the Monoids got invented on a Sunday afternoon, when the designer was halfway down a spliff, but apparently they were invented by Michael Imison, the director, and he certainly doesn't mention a spliff. I think they work really very well.
As does the jungle set in Episode One, complete with lots of live things to look at including an elephant that doesn't disgrace itself like the one on Blue Peter. It's the first thing that really arrests our interest.
The story is intelligently written and well told, and the first two episodes do tell a good tale of a paranoid society and its reaction to the Common Cold - and the resulting deaths, and that truck does good service, and it's nicely acted.
And then the TARDIS returns by accident at the end of the voyage and the Monoids have taken over. Then for the rest of the story, they ill-treat the humans, and scheme against each other, and kill each other, until the invisible Refussians deal with their troublesome bomb by chucking it off the spaceship in one of those moments of deus-ex-machina that speak loudly of a pair of writers (presuming Lesley Scott did contribute to the script) having created a situation that the hero really cannot get out of.
The moral of the story is undoubtedly its strong point, and it's very well played. Invisible monsters always run the risk of seeming an expense-saving cop-out, but Richard Beale does a lovely job of the voice. Michael Sheard and Roy Skelton each do their first Dr Who.
One of the features makes much of the relationship between Dr Who and HG Wells, citing this story as a prime example, and completely ignoring The Unearthly Child, The Daleks and Dalek Invasion of Earth, all of them very clearly Wellsian.