Chameleon (humanoid)

The Chameleons were an advanced humanoid race. An explosion on their home planet resulted in the loss of their faces and their identities. To save their race from dying out, their scientists developed a process that would allow them to assume the identities of other beings. They went to Earth and established Chameleon Tours at Gatwick Airport as a front for abducting young Humans to their space station. By July 1966 they had acquired 50,000 people, but before they could return to their world they were exposed by the Doctor, who offered to help them find another solution to their problem in exchange for releasing the humans. (DW: The Faceless Ones)

Shortly after the Chameleons left Earth, they passed Krennos and decided to kidnap and impersonate its humanoid visitors. After taking their place, the transformed Chameleons would return to their victims' homes for a while to avert suspicion from Krennos, then return to the Chameleon's home world. The Sixth Doctor discovered their plan and put a stop to it. (ST: Face Value)

Technology
In addition to their spacecraft and space station, the Chameleons also had electric guns, laser cannons, and paralysing devices. The process used to assume the identities of Humans involved the use of armband-like devices placed on both subjects. If an armband was removed from the original, the Chameleon that assumed their identity would be killed, reduced to a small blob of protoplasm. The abducted Humans were miniaturised and placed in suspended animation.