This unnamed alien species originally came from a world in the Andromeda Constellation, but they were forced to leave their planet as their sun had become too unstable.
They appear like luminous quasi-humanoid beings and have some form of amorphous biology which allows them to reshape their bodies, turning into a globular mist-like form and taking over the bodies of other creatures to mimic their appearance. They keep their victims in suspended animation within a broadcasting machine in order to remain in their form (very similar to the Zygons). If the machine is suddenly unplugged the alien experiences a great shock, often resulting in its death.
These aliens are much stronger, and implied to be much more long-lived, than Humans. They are more adept at seeing in darkness and are also unharmed by bullets, likely due to their essentially amorphous nature. However, they are also known to be allergic to alcohol, and are lethally vulnerable to concentrated doses of oxygen. When they die they first stop glowing, then their bodies liquefy and dissolve (although, strangely enough, this doesn't seem to happen when they are killed by oxygen).
Members of this species seem to have had some form of sexual dimorphism, and the males and females lived separately from each other, only coming together for mating purposes. Unfortunately, it appears that their females were far more vulnerable to solar radiation, as they all died out before managing to leave the planet, leaving the remaining males alone and unable to have children on their own.
After the females died out, the males left their home galaxy in search for an adequate planet for them to colonize. A small group of them landed on Earth and abducted local men in order to assume their form and take over their place. They were at first unable to interbreed with the Human females, but their scientists eventually managed to work on a way to alter the females' chromosomes and allow genetic compatibility between the two species.
A woman named Marge Farrell (the bride of one of the kidnapped men; who unknowingly married the alien who replaced him) eventually uncovered the whole thing, and after several attempts, managed to convince her physician, Dr. Wayne, of what was happening. Later that night, Wayne gathered a small posse to attack the alien ship in the woods. Although impervious to bullets and armed with disintegrating ray guns, the alien guards were successfully taken down and killed by the group's dogs, and the remaining five aliens (including the fake Bill Farrell) were all killed when the group entered their spaceship and unplugged the broadcasting machine.
Before dying, the alien that had impersonated the police chief sends a message to the rest of the fleet informing them that Humans were dangerous and that they should seek another planet to target. The whole fleet is seen flying away from Earth into the vastness of space, looking for another world in order to try to save their doomed race.
- I Married a Monster from Outer Space (1958)