The inhabitants of the planet Sarn are split into two factions: those, led by the high priest Timanov (Peter Wyngard) and the ‘Chosen One’, who worship the god Logar; and those, led by the ‘Unbelievers’ Amyand (James Bate) and Sorasta (Barbara Shelley), who consider Logar to be a myth. The current ‘Chosen One’ is Malkon (Edward Highmore), found on the mountainside when he was a child, who has a strange overlapping triangle mark on his upper arm. Amyand and his friend Roskal (Jonathan Caplan) climb the fire mountain to see if it is true that, as Timanov preaches, Logar resides at the top. All they find is barren, steaming rock – there is no god. 

On Earth, Professor Howard Foster (Dallas Adams), aided by a team of men led by Curt (Michael Bangerter), retrieves a number of ancient artefacts from the sea bed off Lanzarote. One of these has the overlapping triangle design on it. 

In the TARDIS, a cry from Kamelion (voice: Gerald Flood) brings the Doctor and Turlough running to find the android connected to the ship’s data banks. Turlough goes to the console to program an alpha rhythm to calm the creature. He notices that a distress signal is coming through and recognises it as being of Trion origin. He rips wires from under the console in order to disable it. The Doctor returns and notices that the co-ordinates have been reset – presumably by Kamelion.

Howard’s stepdaughter, Peri (Nicola Bryant), informs him that she intends to travel to Morocco with two English boys. He connives to strand her on his boat but, taking the strangely marked artefact which she hopes might be worth something, she tries to swim to shore. 

The TARDIS arrives on a beach in Lanzarote. The Doctor and Turlough meet up with Howard and Curt. The Doctor thinks that the intermittent distress signal may be coming from the wreck that the two men are scavenging. Turlough returns to the TARDIS to try to triangulate the signal. 

Peri gets into trouble in the water and Turlough, seeing this on the TARDIS monitor, goes and rescues her. He puts her in one of the TARDIS’s bedrooms to recover. The Doctor meanwhile hurries back to the TARDIS and finds the artefact, which Turlough has discovered in Peri’s bag. Turlough also has one of the overlapping triangle marks on his upper arm. 

When Peri dreams of her stepfather, Kamelion takes on his form. The TARDIS then dematerialises of its own accord, and Kamelion-Howard and Peri reveal themselves to the Doctor and Turlough in the console room. 

The TARDIS materialises on Sarn. Its arrival is witnessed by one of Timanov’s lookouts (Simon Sutton), who reports to the high priest that ‘the Outsider’ has arrived. When Turlough and the Doctor leave the TARDIS to investigate, Kamelion changes his appearance to that of the Master (Anthony Ainley). Peri tries to escape, but Kamelion then becomes Howard once more and tells her to find the Doctor. He gives her a component – a comparitor – from the TARDIS. The Master reasserts his dominance over Kamelion and orders the android to release him from his TARDIS, which has just appeared outside. Kamelion and Peri go outside, but an earthquake knocks over the Master’s ship – which is shaped like a column – and Peri is able to escape. 

Turlough explains to the Doctor that the mark on his arm is the Misos Triangle – the symbol of his home planet, Trion. The two time travellers are met by Amyand and Roskal, who take them to their group’s base in an underground control room. The Doctor and Turlough recognise the equipment, and Turlough confirms that it is from Trion. Amyand then takes them to see Timanov. 

Kamelion-Master has meanwhile chased and cornered Peri, but been unable to obtain the TARDIS component from her. He meets Timanov, who assumes that he is a manifestation of Logar. 

In the sanctum, Turlough recognises more components and also a symbol round Malkon’s neck. He thinks that these have come from his own father’s ship, which he and Malkon then rush off to find. Timanov and Kamelion-Master meanwhile arrive and order that the Doctor be burnt. 

Turlough and Malkon find the ship out in the wilderness. Peri also makes her way there, and when she mentions the Master they all rush back to help the Doctor. Turlough operates the machinery in the Unbelievers’ cave to redirect the gas flow from the mountain and cut off the flames, thus saving the Doctor and the other Unbelievers from being burnt alive. Malkon is shot by one of Timanov’s men but manages to relay to the Doctor that the Master is Kamelion. The Doctor confronts the android but the Master manages to keep control and has the Doctor locked in the fire cave. Kamelion-Master and Peri then leave, and Turlough is able to release the Doctor. Turlough now thinks that Malkon is his brother. 

With the assistance of Timanov and a group of Sarns, Kamelion-Master has the Master’s TARDIS set upright. The android and Peri then enter the ship. The Doctor’s intention to move his own TARDIS around the Master’s is foiled as Kamelion has taken his temporal stabiliser. However, the navigational systems of the Doctor’s TARDIS are locked onto the Master’s ship. Kamelion-Master moves the Master’s TARDIS to a seismic control room at the heart of the volcano, where he explains to a bemused Peri that numismaton gas is a rare catalytic reagent from inside the planet. Knocking Kamelion-Master over, Peri runs back into the Master’s TARDIS, where she investigates a box which she assumes controls the android. Inside is a miniaturised Master and a control room. 

The Doctor, realising that the fire cave walls are coated with numismaton gas residue, re-diverts the gas there. When the flames burn blue, Malkon is taken into them and cured. The Doctor and Amyand go to the seismic control centre, while Turlough and Malkon place a distress call to Trion from the crashed spacecraft. Turlough reveals that the Misos Triangle denotes a prisoner – Sarn is where Trion prisoners are sent. He explains that there was civil war on his planet and his mother was killed. His father and brother were sent to Sarn, while he was sent to public school on Earth. 

The Doctor and Amyand meet Peri in the hot, lava-runnelled wilderness and return with her to the cave. The Doctor takes the temporal stabiliser from the Master’s TARDIS and sends Amyand to give it to Turlough. The Master reveals that his miniaturised state is due to an accident he suffered while building a more deadly version of his tissue compression eliminator. He revives Kamelion and, when the Doctor and Peri are distracted, the android moves the Master’s TARDIS to the column of flame. Kamelion places the box containing the shrunken Master in front of his TARDIS. The Doctor and Peri disable Kamelion and, at the android’s own request, the Doctor destroys it using the Master’s eliminator weapon. 

Amyand gives Turlough the component and he fits it into the TARDIS, which leaves of its own accord to follow the Master’s ship. 

The flame in the seismic control room turns blue and the Master grows to his normal size. However, the flame then turns back to orange and the Master vanishes in a puff of smoke, screaming in pain. As the control centre explodes, the Doctor and Peri leave in his newly-arrived TARDIS. 

A Trion ship has arrived to ferry the survivors off the doomed planet. Turlough decides to go back to his home with the Trion Captain, Lomond (John Alkin), now that political prisoners are no longer persecuted. The Doctor bids him farewell. 

Peri decides that she doesn’t want to be returned home: she wants to travel and still has three months of her vacation left. The Doctor agrees that she can stay with him.

_{Synopsis from Doctor Who: The Fifth Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}