The Chameleons are a sapient, technologically advanced humanoid species which at some point suffered an unspecified disaster which caused them to become degenerate beings with no facial features and no identities.
The exact details of this transformation are up to speculation, but it threatened the race with extinction. In order to save themselves, the Chameleons secretly infiltrated in Earth society and devised a method that allowed them to extract genetic material from other beings and essentially "steal" their appearances and identity (although retaining the memories and personality of the Chameleon), allowing them to regain genetic stability. The process involves placing armband-like devices in the arms of both subjects: donor and receiver. It takes some time to become permanent, during which time the armband cannot be removed from the Human subject, otherwise it would kill the Chameleon.
The Chameleons regard themselves as the most intelligent life forms in the universe (although they don't appear to be familiar with Time Lords and are impressed by the Doctor's intelligence) and claim that in terms of intellect, Humans are comparable to the animals of their own planet. Their technology also includes means to make an airplane harmlessly leave the atmosphere into space, and the ability to miniaturize people and place them in suspended animation.
Disguising themselves under the guise of the Chameleon Tour airlines which they founded, the Chameleons have secretly abducted hundreds of Human beings in the year 1966, and brought them to a space station, planning to take them to their homeworld to serve as identity donors to the Chameleon population. This plan was discovered and stopped by the Second Doctor, who offered to help the Chameleons find another, more peaceful solution to their problem.
- Doctor Who season 4 (Second Doctor) - The Faceless Ones (1967)