On the desert world of Sarn, robed natives worship the fire god Logar and follow the Chief Elder, Timanov, who demands obedience. Those who dissent are known as Unbelievers, and two of them, Amyand and Roskal, cause unrest when they claim to have ventured to the top of the sacred fire mountain but not found Logar. One of the Sarns, Malkon, is known as the Chosen One because of the unusual double triangle symbol burnt into his skin: he is also unusual for having been found as a baby on the slopes of the fire mountain.
The same triangle symbol is found on a metal artefact uncovered in an archaeological dig in Lanzarote overseen by Professor Howard Foster. His stepdaughter Peri Brown is bored with the dig and wants to go travelling in Morocco and when he seeks to prevent this she steals the strange artefact and tries to swim for freedom. Fortunately for her the TARDIS has landed nearby – responding to a distress call sent by the strange artefact - and Turlough sees her drowning and rescues her. Going through her possessions as she recovers he finds the artefact and acknowledges the same triangle symbol is burnt into his own flesh. The Doctor returns to the TARDIS after attempting to triangulate the source of the signal being emitted by the artefact, and the ship dematerialises, seemingly on its own. It soon arrives on Sarn and the Doctor and Turlough set off to explore.
The Doctor's other companion, the android Kamelion, has meanwhile made mental contact with its old controller, the Master, who attempts to assert his control and change Kamelion's appearance from that of Howard. Kamelion tries to warn Peri of the Master but the Time Lord succeeds in gaining control. She flees the TARDIS with the creature in pursuit as the rumblings of the volcanoes of Sarn gather ferocity.
In the Sarn colony Timanov has damned the Unbelievers to be sacrificed to appease Logar and stop the tremors. They flee to a secret base in the mountains which is filled with seismological apparatus and which the Doctor and Turlough stumble across. The Doctor informs the Unbelievers that the tunnels which have been their refuge are volcanic vents which will soon fill with molten lava. It is also established that Turlough is of the same race as those who colonized the planet, and when the indigenous people see his Misos Triangle, they greet him as a second Chosen One. Turlough realises Malkon may be his brother and becomes even more worried when Peri turns up and mentions the Master.
Another important figure in Sarn mythology is the Outsider, a promised prophet, and the Master/Kamelion fulfils this role admirably. He convinces Timanov of the appropriateness of harsh action and when the Doctor arrives with the Unbelievers they are all seized for burning. However, Malkon and Peri arrive shortly afterward and end this assault, though not before Malkon has been injured. Turlough is aghast when he finds his relative has been shot and the Doctor presses him for as much information as he has on the strange circumstances of Sarn. It seems it is a long abandoned Trion colony planet, and that Turlough, a Trion, suspects some of his family were sent here after a revolution against the hereditary leading clans of his homeworld. He supposes his father died in a crash but that Malkon survived, while he himself was sent in exile to Brendan School In England, overseen by a Trion agent masquerading as a solicitor in Chancery Lane.
The Master/Kamelion has meanwhile seized Peri and uses her to transport a black box into the control room of his TARDIS. It contains a miniature Master – the real thing – who has been shrunken and transformed by a disastrous experiment with his Tissue Compression Eliminator. The Master thus re-established the psychic link with Kamelion to gain the power of movement and has manoeuvred the robot to Sarn so that he can take advantage of the restorative powers of the Numismaton gas within the fire mountain.
Turlough realises the imminent volcano bursts will destroy the Sarn colony so nobly uses a functioning communication unit to get in touch with Trion and plead for a rescue ship to evacuate Sarn. In doing so he abandons his own freedom. When the ship later arrives, the Sarns all depart, along with Turlough and Malkon. They have both been pardoned in an amnesty issued by the new Sarn government. The only one to remain on the planet and face the erupting volcanoes is Timanov, now sure to die, his faith in tatters.
The Doctor meanwhile succeeds in weakening the Master's hold of Kamelion, and interrupts the numismaton experiment. He adds calorific gas to the numismaton surge and seemingly burns the Master alive. The Doctor also puts the terminally wounded Kamelion out of its misery. He returns to the TARDIS with a heavy heart, but with a new companion, Peri, for company.
Doctor Peter Davison (Fifth Doctor)
Companions Mark Strickson (Vislor Turlough)
Gerald Flood (Kamelion)
Nicola Bryant (Peri Brown)
Anthony Ainley — The Master
Dallas Adams — Howard
Michael Bangerter — Curt
Peter Wyngarde — Timanov
Barbara Shelley — Sorasta
James Bate — Amyand
Jonathon Caplan — Roskal
Edward Highmore — Malkon
Max Arthur — Zuko
John Alkin — Lomand
Simon Sutton — Lookout
Writer Peter Grimwade
Director Fiona Cumming
Script editor Eric Saward
Producer John Nathan-Turner
Executive producer(s) None
Production code 6Q
Series Season 21
Length 4 episodes, 25 minutes each
Originally broadcast February 23–March 2, 1984