On the desert world of Sarn, robed natives worship the fire god Logar and are led by Timanov, the Chief Elder (Peter Wyngarde). Two unbelievers, Amyard (James Bate) and Roskal (Jonathan Caplan) have climbed the Sacred Fire Mountain and did not find Logar there. Another, Malkon (Edward Highmore) who was found as a baby on the slope of the mountain, has a double triangle burnt into his skin.
The same symbol appears on an artefact uncovered in an archaeological dig in Lazarote bry Professor Howard Ester (Dallas Adams). His stepdaughter Perpugilliam Brown, called Peri (Nicola Bryant), is bored by archaeology and wants to leave to travel to Morocco, but has no money. She takes some from her stepfather’s wallet and takes the artefact too, which may be valuable, and swims to shore. The TARDIS lands nearby and Turlough saves her from drowning. As she recovers in the TARDIS, he finds the artefact and the symbol, which matches the one burnt into his arm. The Doctor returns and the TARDIS dematerializes on its own, landing on Sarn. The Android Kamelian has been contacted by the Master (Anthony Ainley). Kamelion tries to warn Peri, but the Master takes control. She runs from the TARDIS and finds herself surrounded by rumbling volcanoes.
In the Sarn caverns, Timanov wants to sacrifice the unbelievers to stop the tremors. They escape into the mountains, where the Doctor and Turlough find seismological apparatus. The Doctor tells the unbelievers that the tunnels are volcanic vents and will soon be filled with lava. It turns out Turlough is of the same race as the colonisers of the planet, and seeing his triangle mark they worship him as the Chosen One. Malkon may be his brother, sent here when Turlough was sent to Earth.
Peri turns up and mentions the Master. The Outsider is another figure in Sarn mythology, and the Master fulfills this role as far as Timanov is concerned. When the Doctor arrives with the unbelievers, Timanov convinces the people to burn him as well, egged on by the Master. Malkon arrives to put a stop to this but is injured. Sarn is a Trion colony of old, and Turlough believes that it was used as a prison for Trion rebels, who were branded with a triangle mark.
The Master has seized Peri and uses her to place a black box in the Master’s Tardis, containing the Master himself in miniature, trapped by the failure of his Tissue Compression Eliminator. He has used Kamelion to bring him to the restorative powers of the Fire Mountain’s Numismaton Gas.
Turlough knows the volcanoes will destroy Sarn, so he uses a comm unit in the crashed Trion spaceship to contact Trion and ask for the people to be rescued, though he himself risks prison. He rescues the Doctor and Peri. Only Timanov and the oldest believers refuse to be evacuated. The Doctor weakens the Master’s power over Kamelion. The Master is trapped and appears to be burnt to death in the formerly restorative fire. Kamelion is terminally damaged and must be disconnected. Turlough learns that an amnesty is in effect on Trion; he no longer need fear arrest, and he leaves with the evacuees. The Doctor escapes from the volcanoes with Peri in the TARDIS, and she decides to travel with him for a while, at least for the rest of her vacation.
The Doctor lost his Cricket outfit for this story, as a sweater would have seemed ridiculous in the sun-baked landscape—Lanzarote in the Canary Islands. Turlough lost his school-boy uniform and tie. Peri’s pink bikini is a popular image in the annals of Doctor Who. Peter Wyngarde, best known for the eccentric detective in the British Jason King series, both looked and sounded impressive; in his desert robes, he reminded me of Peter O’Toole in Lawrence of Arabia. The ill-stared robot Kamelion had not appeared since The King’s Demons the year before. Presumably it was sitting idle and half-broken in the TARDIS all this time. We finally learn Turlough’s mysterious backstory. The Fifth Doctor is down to one companion in his next-to-last story. She’s certainly attractive, but her constant whining quickly becomes tedious. She is the British idea of a spoiled American brat.