Leonard Maltin called INVISIBLE INVADERS "cheap, silly, and boring." I suspect that Maltin's reaction is largely informed by the cheapness of this Edward L. Cahn film, especially with respect to its copious use of stock footage, but there are some sociological aspects of INVADERS that aren't altogether "silly."
In my review of THE 27TH DAY I disliked that film's mixed message, in that the story began by castigating mankind for its warlike ways but ended by making it okay to kill off only "the bad people" in the world. In contrast, INVADERS is indisputably a "hawkish" film, where the only advocate of peace is fundamentally proven to be wrongheaded.
The film proper begins following a voiceover-segment establishing the continuing conflicts of Earth's cultures in the race for "atom supremacy." When one particular atom-scientist, a Dr. Noymann (John Carradine), perishes in an explosion, his friend and colleague Adam Penner becomes disenchanted with the costs of the atomic effort. Penner seeks out a ranking general heading a US atomic commission, and tries to persuade the general to help institute a movement to use atomic energy only for peaceful purposes. The general, wholly committed to protecting his country, refuses to compromise US security in any respect, so Penner resigns from his position with the commission.
Shortly thereafter, Penner learns that the general is right to insist on military supremacy. A resurrected Dr. Noymann appears at Penner's house, asserting that he has been reanimated by one member of an invisible alien invasion-force. "Noymann" tells Penner to persuade his people to surrender all authority to the aliens, or they will wreak destruction upon the human species. Penner patriotically declares that the human race will fight back no matter what, but after the alien zombie departs, Penner can't even convince his daughter Phyllis or her "old friend" (read: former boyfriend), scientist John Lamont, to believe what's happened. Slightly later, however, all three of them witness another alien manifestation in a graveyard, where they witness how the invisible beings leave tracks in the dirt-- indicating that they apparently have mass, but somehow become insubstantial as they merge with corpses.
Since Penner's warnings are ignored, the aliens make good on their threats, unleashing an army of unkillable zombies who create all manner of stock-footage havoc. Perhaps it's a measure of the military hierarchy's embattlement that when it chooses to believe Penner and his allies, it assigns only one soldier, Major Bruce Jay (John Agar) to protect them as they search for the aliens' scientific Achilles heel. Of course, it's more likely Cahn just didn't want to pay for enough actors to play a full detachment of soldiers. At one point, when Jay is conveying his three charges to a scientific installation, their jeep is stopped on the road by a shotgun-wielding farmer out to steal their vehicle. However, Jay beats the farmer to the draw, though the three civilians are initially repulsed by the killing of a relative innocent.
In due time Penner and Lamont do come up with the necessary Achilles heel after managing to capture one of the alien-possessed zombies, and Earth is saved by the use of scientific intelligence as directed by military acumen.
In addition to the film's quick dismissal of Penner's initial ethical stance, its sentiments are made even plainer from a subplot in which Penner's daughter Phyllis falls in love with the no-nonsense major, who's a "real man" as opposed to the squeamish Lamont (though Lamont does at least hold his own for a while when he and Jay come to blows over Phyllis). Though Lamont isn't a peacenik like Penner, he remains in Penner's shadow by virtue of having been Penner's student, whereas Jay is consistently courageous, risking his life to bring back an alien specimen.
The makeup for the zombies is in some instances rather grotesque for the period, emphasizing the bloody and mangled look of the risen dead, lending a sense of apocalyptic doom to the stock-footage violence. And in contrast with Cahn's CREATURE WITH THE ATOM BRAIN, which shares a 'scientific zombie' theme, there's always something happening in INVADERS, cheap as the proceedings may be.