Invisible Invaders was made in 1959, and it shows, but it was still a fun movie. Like many horror movies in the 1950s, Invisible Invaders has a heavy-handed anti-nuclear weapons, why-can't-we-all-just-get-along message, but the camp can still make you laugh, and there are some well-done zombies.
The movie starts with the death of Dr. Karol Noymann, played by B-movie regular John Carradine, in a radioactive explosion at the lab. His death makes his friend and colleague, Dr. Adam Penner (Philip Tonge), realize the evils of nuclear weapons, and Dr. Penner decides to abandon his research. The night after the funeral of his friend, Dr. Penner is visited by the corpse of Dr. Noymann, animated by an alien who tells him that because the human race has developed atomic energy, they now have 24 hours to surrender to the aliens or be exterminated.
When the aliens aren't inhabiting corpses, they're invisible, as are their ships, making them very difficult to fight. As the Earth is being overrun, saving the human race is up to Dr. Penner, his daughter Phyllis, their uptight and panicky friend Dr. Lamont, and Air Force Major Bruce Jay, charged with protecting them and played by another B-movie regular, John Agar.
The science and logic in the film are a bit shaky, but that isn't all that surprising. The acting is just the right combination of ham and cheese to be entertaining, so if you turn your brain off, you can probably enjoy yourself.
I thought it was interesting that this movie came out a year after Plan 9 from Outer Space. Look at the similarites: aliens, concerned by our developing atomic power, come to Earth and reanimate our dead in order to get us to stop (not to mention that both movies start off with the death of a character played by an aging horror star who comes back as the first zombie).