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.