Zygons

Shape-changing aliens which arrived hundreds of years ago on Earth. After sending a distress call, their leader, Broton, learned that their homeworld had been destroyed in a natural disaster.

He suggested that the Zygon fleets come to Earth and although it would be many hundreds of years before they arrived, he planned to terraform Earth in preparation.

The Zygons brought with them a Skarasen which supplied them with the nutrients they needed to survive. By the late twentieth century, Broton was ready to begin his plan.

He replaced the Duke of Forgill, an important local to the people of Tullock, as well as being on the board of the Scottish Energy Commission. This gave him access to the World Energy Conference, to which he planned to lure the Skarasen and use it to destroy everyone inside, throwing the world into panic and confusion.

The other Zygons also impersonated Tullock locals. One, Madra, briefly impersonated Harry Sullivan but was killed trying to attack Sarah Jane Smith.

The other two impersonated Sister Lamont from the hospital and the Duke's ghillie, known as the Caber. They were blown up when the Doctor activated the ship's self-destruct mechanism, while Broton was shot by Brigadier Lethbridge-Stewart.