The Invaders by Quinn Martin with Roy Thinnes was successful because, in my opinion, we never saw the true form of the alien beings from a dying world.....
There were also certain rules that we learned about what they were and their terrestrial limitations. We learned, for example, that they had taken human form and had to remain in that form while on Earth or they would die. They had been here for untold years before David Vincent's fateful encounter with them one dark night on a lonely road at a deserted diner, thus they had infiltrated positions in society from a prostitute in a bar to positions of power within the Department of Defense. They could be men, women, and even children. Paranoia was rife because one could not be certain that anyone from the paper boy on his bike to a high ranking politician on telly was human. In every way they looked and behaved like normal human beings.
We also learned that the aliens did have limitations that could reveal their identity. Though they had the outward appearance of being human, they were very different inside: They had an irregular skeletal structure, no heart, and no blood; when cut, they would not bleed. An X-ray, blood test, or any number of medical procedures would quickly disclose their less than human nature, however a simple test was just to take a pulse which would give the appearance that they were dead even though they were fully conscious and ambulatory.
They had to regenerate periodically to remain in human form. Only in human form could they breath our weak 20% mixture of oxygen for limited periods of time before they would have to regenerate; pure oxygen would bring on an instant requirement for regeneration. They knew their limitations and would regenerate long before the need became critical. When, on occasion, the need became critical, they would begin to return to their alien form, but since they could not live on Earth in their native form, they would die before the transformation was complete. Either by design or happenstance, they would incinerate at the moment of death in our atmosphere. Before they could turn to their native form, they would begin to glow (an indication that they were dying and that incineration was imminent). Thus they could be killed as easily as any human, but since David Vincent's need was not just to kill them individually, but to have a living specimen to prove to a disbelieving world that they were here and the nightmare had begun, and since their dedication to their purpose and anonymity was greater than their fear of death, he just could never bring off a perfect disclosure.
IN THE FILM VERSION, we were given the hype that this would answer the question of what became of the aliens and David Vincent in the quarter century since the show was terminated without catharsis. The reality was that the only hint of a tie-in was the presence of David Vincent for what amounted to a cameo appearance in the first and second part. They did not have the polished human appearance of those in the series; they looked and behaved like whacked-out zombies and instantly drew suspicion that all was not well. The rules were all changed and they made the mistake of trying to show the aliens in their native form. This killed the mystique and violated the principle that THERE IS NOTHING THAT THE EYE CAN BEHOLD THAT IS AS HORRIFIC AS THAT WHICH CAN BE CONJURED IN THE MIND. With the series, we each had a mental vision of what they looked like that was far more terrifying than the Hollywood magicians could give us at their best, let alone the 1950s style, rubber masked freaks that were offered here.
Artistic infidelity is not just limited to book to film translations. The film version of the classic television series of The Invaders shows that in various media, some things do not lend well to artistic license and should be left in the form for which they were designed.
