The Extractor was an artificial life-form built and controlled by an alien race. It looked human except for its blubbery, gelatinous face.
It was a mould filled with a smoke-like substance that would be exhaled through a long thin white tube in the mouth which resembled a cigarette. The smoke would extract the memories of any person it contacted, leaving them almost completely blank slates, then beam the memories to a nearby spacecraft. The aliens would thus gather information without having to figure things out for themselves. If an Extractor was damaged and/or lost power, it would eventually decompose into nothingness, leaving only its clothes behind.
In the 1970s, an Extractor took the minds of four Russian cosmonauts just before their return to Earth, but when the capsule arrived, all that was left of it was an empty spacesuit. Another Extractor arrived on Earth and took the minds of a bus driver and a policeman. It then attempted to take the mind of the Third Doctor, who successfully resisted its efforts.
Almost exhausted, it then tried to take the mind of Liz Shaw, but was unable to finish its work. The next day, the Doctor discovered the Extractor's shrivelled body and took it to his lab to analyse it. After returning the amnesiac Liz to the hospital, he returned to the lab to find the Extractor recharged. It tried again to take the Doctor's memories, but the Doctor narrowly managed to escape and locked the lab door, trapping the Extractor until it lost power and decomposed. (PROSE: The Mind Extractors)