Garlin was a being that lived in the stone circle near RAF Lossiemouth. He rescued Mike Yates from the stones as he would have been atomised. He came from a planet called Pharas and was surveying the planet, similar to his own to look for resources. His race can manipulate igneous rocks.
Garlin created many stone circles in the region and used to created data collectors. Granite and serpentine worked quite well. He wanted to judge the humans for their actions 4000 years ago. The Third Doctor told them not to and they agreed until Sgt John Benton and his UNIT troops attacked. He then wanted to destroy the humans because of this. After the Doctor foiled his plans, Garlin decided to kill the Doctor by dragging him in to the recumbent stone. (AUDIO: Vengeance of the Stones)