The Doctor and Turlough arrive on Lanzarote to investigate a signal. Turlough rescues American Student Peri Brown from nearly drowning and discovers she has in her possession a data core from Turlough's home planet Trion. The Doctor, Turlough and Peri follows the signal to the volcanic planet Sarn, where the natives worship a fire god known as Logar, where the Doctor learns The Master has accidentally shrunk himself after testing a more lethal version of his tissue compression eliminator, and he has taken control of Kamelion and he is using Kamelion to find numismaton gas that will will restore him to his normal size.