Like many low budget horror films of the era, the first two acts are pretty boring and insubstantial. When many of those films finally revealed the monster, however, it was laughably goofy and anticlimactic. The monsters in this one, though, are like Ray Harryhausen-meets-David Lynch and the spectacle of their violent destruction is worth the wait.
"An invisible threat" opens a lot of allegorical avenues, and this one makes sure to really state what the concern is (nuclear power), but so much of its admittedly short run time is a drag. I did like the satellite precursor to the "enhance!" phenomenon - this machine even lets you know when you're approaching the danger zone