Second voice

An alien, referred to as the second voice by the Third Doctor and Jo Grant, was present when Thomas Edison made the first audio recording in history, a reading of Mary Had a Little Lamb. When Dr Van Mayer of the Lezarata Research Centre developed a method of giving consciousnesses to recorded voices, he tested it out on the recording of Edison, accidentally bringing the second voice to life as well. It lived inside the recording, and one by one trapped all the residents of the research centre on the tape with it. However, Van Mayer ordered the base to be locked from the outside, trapping the alien inside along with all the scientists who worked there.

Decades later, the Third Doctor and Jo Grant landed in the research centre. The alien saw the Doctor's TARDIS as a way to escape, and trapped them on the tape, possessing Jo's body. It soon realised, however, that only Jo and the Doctor knew how to get into the TARDIS, so it let Jo — or who it thought was Jo — escape, and threatened her into letting it in. However, unbeknownst to the alien, Jo and the Doctor had switched bodies while they were both trapped on the recording, and the Doctor was able to outsmart the alien and trap it on a tape doomed to be erased. (AUDIO: Ghost in the Machine)