The short-lived Planet Productions managed to get the great Terence Fisher and Peter Cushing together again (like 'Island of Terror' a year earlier). Even Christopher Lee was persuaded to take part in this science-fiction flick!
This effort was actually filmed in February/March 1967 and, as such, would not have been a particularly comfortable film to make (the cast were covered in glycerine to create the sweaty effect!).
Fisher manages to create a suitably isolated feel to the whole film - the inhabitants, who literally "live in their little old world" are cut off from the rest of civilisation.
However, as with films of this kind, the low budget decidedly restricts the scope of the film. There is a ton of exhausting dialogue at the inn and the characters are not particularly interesting.
Peter Cushing's character of Dr. Stone gets very little screen time and it is strange not to see Cushing at the climax of the film!
Christopher Lee walks around very sternly, not particularly masking his discomfort well at appearing in such a film.
The creatures responsible for causing the heatwave in the film are cheap and nasty creations, that do nothing to enhance the credibility of the film.
Ultimately, despite the fact that the film is well-intentioned, it is far from Peter Cushing/Christopher Lee's best!