Liz Shaw's friend, Jean has gone missing and her cat has been killed by something, so the Doctor and UNIT are called in to investigate Jean's disappearance, as well as the deaths of scientists from Cambridge. After finding some teethmarks in Jean's cottage, they find a small tunnel in the garden. They learn that all the people who have died all had the same dentist, Gareth Arnold who worked at the University of Cambridge.
After being attacked by Cybermats that have emerged from a dead body they were investigating, the Doctor realizes that they are dealing with the Cybermen and that Liz has had some of Gareth Arnold's blue filling placed in her mouth.
Tropes:
- Body Horror: The Cybermats inside Andrew Cull's dead body. They appear because of the blue filling that Gareth Arnold uses on his patients. Once it's activated, it consumes the victim's body and uses it to create more Cybermats.
- Depraved Dentist: Gareth Arnold. He operated at Trumpington Street Surgery until he found a crashed cyber ship and experimented with the surviving technology, resulting in him creating the blue filling that causes Cybermats to be created inside his patients' bodies.
