The Zeld were an advanced humanoid race. They were capable of creating energy-based Proto slaves and flying saucers which could fly through solid granite. In the 1970s, a Zeld scout ship came to Earth and made its base in Scotland. To refuel for the trip home, they captured humans from all over the world and drained them of their psychic energy. They also drained nuclear energy from the Grestonspey power station. When UNIT investigated, they captured the Third Doctor, but he escaped and advised UNIT to attack the Zeld base. (PROSE: The Ghouls of Grestonspey)