Unit has been called to assist a recent project designed to drill through the Earth's crust to discover a new energy source. The Doctor doesn't think it's such a good idea, but no one else seems to listen to his warnings. When a strange green ooze is discovered by a technician, instead of reporting it he... you guessed it - touches it. It immediately affects him and he begins to change and turns into a creature known as a Primord.
As the Doctor's precautions go unanswered he decides to work on the Tardis. He's removed the console and is attempting to repair it, in hopes of traveling once more. He tells Liz he feels trapped. With her help, the console works and the Doctor disappears. But it looks like he hasn't moved at all and Liz is not in the room with him. Going outside he's immediately shot at by one of the UNIT guards.
Instead of traveling in space or time, the Tardis has actually gone sideways into a parallel dictatorial world. He meets Section Leader Liz Shaw and the Brigade Leader Lethbridge-Stewart who do not know who he is and believes he's a spy. He's interrogated and threatened with execution. He's ultimately thrown into a prison cell where the Doctor makes a startling discovery.
Though this world is running parallel, it's also slightly ahead in time. He finds out what the ooze is turning people into and that penetrating the Earth's crust will result in the destruction of Earth. His warnings are again ignored, but he is able to convince the UNIT counterparts to help him get back to his Earth so he may warn them. Reluctantly, they agree in face of their own deaths.
The Doctor arrives back to his Earth before penetrate of the Earth's crust but project director Stahlman sabotages the equipment so the drilling does not stop. Having come into contact with the ooze he also changes into a Primord. The Doctor convinces the staff, and the Brigadier, of the dangers and they all help to shut the operation down.
WHO: Elizabeth "Liz" Shaw
DEPARTURE LOCATION: Earth, Circa 1970
REASON FOR DEPARTURE: Though she is never given a proper exit story, it's explained in the next story she returns to Cambridge. In actuality, the producers decided against renewing her contract because they wanted someone not as intelligent as the Doctor to give him someone to explain things to, for the sake of the audience. Ironically, Carol John was pregnant and was about to inform the producers she would not be returning.
NEXT: Master Grant