the invader by quinn martin with roy thinnes be successful because and in my opinion and we never saw the true form of the alien being from a die world . there be also certain rule that we learn about what they be and their terrestrial limitation . we learn and for example and that they have take human form and have to remain in that form while on earth or they would die . they have be here for untold year before david vincent fateful encounter with them one dark night on a lonely road at a deserted diner and thus they have infiltrate position in society from a prostitute in a bar to position of power within the department of defense . they could be men and woman and and even child . paranoia be rife because one could not be certain that anyone from the paper boy on his bike to a high ranking politician on telly be human . in every way they look and behave like normal human being . we also learn that the alien do have limitation that could reveal their identity . though they have the outward appearance of be human and they be very different inside represent they have an irregular skeletal structure and no heart and and no blood while when cut and they would not bleed . an x ray and blood test and or any number of medical procedure would quickly disclose their less than human nature and however a simple test be just to take a pulse which would give the appearance that they be dead even though they be fully conscious and ambulatory . they have to regenerate periodically to remain in human form . only in human form could they breath our weak 20 % mixture of oxygen for limited period of time before they would have to regenerate while pure oxygen would bring on an instant requirement for regeneration . they know their limitation and would regenerate long before the need become critical . when and on occasion and the need become critical and they would begin to return to their alien form and but since they could not live on earth in their native form and they would die before the transformation be complete . either by design or happenstance and they would incinerate at the moment of death in our atmosphere . before they could turn to their native form and they would begin to glow ( an indication that they be die and that incineration be imminent ) . thus they could be kill as easily a any human and but since david vincent need be not just to kill them individually and but to have a living specimen to prove to a disbelieving world that they be here and the nightmare have begin and and since their dedication to their purpose and anonymity be great than their fear of death and he just could never bring off a perfect disclosure . in the film version and we be give the hype that this would answer the question of what become of the alien and david vincent in the quarter century since the show be terminate without catharsis . the reality be that the only hint of a tie in be the presence of david vincent for what amount to a cameo appearance in the first and second part . they do not have the polished human appearance of those in the series while they look and behave like whack out zombie and instantly draw suspicion that all be not well . the rule be all change and they make the mistake of try to show the alien in their native form . this kill the mystique and violate the principle that there be nothing that the eye can behold that be as horrific a that which can be conjure in the mind . with the series and we each have a mental vision of what they look like that be far more terrifying than the hollywood magician could give u at their best and let alone the 1950s style and rubber mask freak that be offer here . artistic infidelity be not just limit to book to film translation . the film version of the classic television series of the invader show that in various medium and some thing do not lend well to artistic license and should be leave in the form for which they be design .