It is the artefact that is emitting the signal, leading them all to the planet Sarn, a world ravaged by volcanoes and yet holding the secret of eternal life. But an old foe of the Doctor's is desperate for this elixir, and will stop at nothing to gain it.
What is Turlough's link to the artefact? What is the nature of Sarn's mythical god, Logar, and how is it linked to the ruined spaceship crashed on a mountainside? As age-old systems of belief begin to break down, will anyone escape from the planet of fire with their life?