The Investigator is an alien being who recruits and miniaturizes a pair of Earthlings to act as his agents in a mission to make the world a better place. Sylvia Anderson wrote the script, with Gerry directing, and also voices Julie, one of the two agents.
This would've been better had it been done entirely with human actors. The fact that Julie and her partner, John, were already shrunken as this story begins tells us that there is more to this than had been told.
Could it be done today with modern technology? I think so. An all-human cast with CGI effects makes more sense.