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 follow the signal to the volcanic planet Sarn, where the natives worship a fire god known as Logar, and where the Doctor learns The Master has accidentally shrunk himself after testing a more lethal version of his tissue compression eliminator and has taken control of Kamelion, using him to find numismaton gas that will restore him to his normal size.