The people of the planet Sarn are approaching a moment of crisis as their god Logar tests their faith. The fire mountain above their settlement is stirring, and soon boiling seas of lava will pour forth and all will be laid waste. Timanov, Chief Elder of the Sarns, insists that with faith in Logar they will survive and be blessed with gifts from the legendary Outsider; but the young Chosen One, Malkon, is plagued by doubt and odd dreams almost like memories. Nevertheless, he was found on the slopes of the fire mountain with the double-triangle sign of Logar burned into his flesh, and as such must be the messenger of the god. But he and Timanov are soon to face their greatest crisis of faith; two Unbelievers, Roskal and Amyand, have climbed the slopes of the fire mountain and found nothing at the summit -- no god, no machines, just an empty volcanic crater. Meanwhile, on Earth, archaeologists studying a sunken Greek galleon in the sea off Lanzarote find something odd amongst the artefacts -- a metal cylinder with a double-triangle symbol carved into the end...

The Doctor's brooding about Tegan's departure is cut short when he and Turlough hear Kamelion screaming in agony. They find him plugged into the TARDIS data banks, and going into spasm for some reason; the Doctor stays with him while Turlough programmes a soothing alpha rhythm into the console. As he does so, he picks up a distress call -- and to his horror, recognizes it as a call from a Trion craft. He sabotages the communications console, claiming that it was picking up random emissions which might have been interfering with Kamelion. But it's too late; Kamelion has coverted the signal into co-ordinates and sent the TARDIS to the source of the distress call.

As Professor Howard Foster supervises the offloading of artefacts from the galleon, his step-daughter Peri Brown arrives to announce that she's had enough of this boring island and has decided to go to Morocco with a couple of English guys she met at the hotel. Howard is unable to talk her out of her decision, but convinces her to accept a loan from him rather than cash in her return ticket to New York. He convinces her to go back out to the boat, where he left his wallet -- but abandons her there so she will miss her flight. Determined to have her own way, she takes the unidentifiable metal cylinder, hoping it's worth something, and tries to swim back to the beach.

The TARDIS materializes on Lanzarote, where the Doctor, his curiosity piqued, tries to pinpoint the source of the signal. He and Turlough see the archaeologists and theorize that they may have disturbed something alien. Turlough offers to return to the TARDIS to help triangulate the signal, but once there he finds Kamelion trying to dematerialize and "make contact". Turlough uses Kamelion's link with the TARDIS to send the robot into spasm, determined to prevent the Doctor from locating the Trion ship. But before he can do anything further he sees Peri on the scanner, suffering from cramp and starting to drown. He reluctantly swims out to her rescue and takes her into the TARDIS to recover, but as she passes out he notices the cylinder she's carrying and recognizes the mark on its side. As well he should; the same double-triangle symbol is burned into the flesh on his arm.

Turlough returns to the console room to destroy the data core, but Peri starts to dream, apparently about Howard punishing her for her misdeeds -- and in a nearby room, the spasming Kamelion picks up on her emotions, uses them to stabilise himself in the shape of Howard Foster, and plugs himself back into the TARDIS data bank. The Doctor arrives, having picked up the signal from within his TARDIS, to find Turlough holding the data core which he claims to have picked up on the beach. The Doctor tries to connect it to the TARDIS console, but a sudden power surge fries the data core -- and causes the TARDIS to dematerialize. "Howard" then enters the console room, claiming to have seen Turlough rescue Peri -- and the confused Peri also arrives. The Doctor apologises and promises to return them to Earth as soon as possible, and as Peri tries to figure out what's going on, the TARDIS materializes on Sarn.

The Sarns have gathered in a communal meeting hall, where Timanov and Malkon urge them to prove their faith by remaining in the settlement during the Time of Fire. But the Unbelievers wish to evacuate -- and then Amyand and Roskal arrive, claiming to have trespassed on the sacred mountain and seen nothing. As Timanov's standing begins to wane he prays for a sign -- and in an unfortunate coincidence of timing, a fresh quake rocks the settlement and the sacrificial chamber bursts into flame. Malkon is now faced with the unpleasant duty of ordering the burning of the Unbelievers... but before he can do so, a lookout who saw the TARDIS' materialization arrives to announce that the Outsider has come!

The Doctor and Turlough find the double-triangle mark amongst the ruins of an old colony, and Turlough identifies it as the "Misos Triangle" -- proof that this is an old colony of Trion, his home world. As they set off to search for survivors, the Master's TARDIS materializes nearby in the shape of a stone column. Meanwhile, Malkon releases the Unbelievers, claiming that the Outsider's arrival proves that Logar does not require sacrifices. Timanov sets off to greet the Outsider in person, and orders Malkon to steel himself for the sacrifices which must follow; Timanov blames the Unbelievers for the current crisis, certain that it is their doubt which has brought down Logar's wrath. Amyand, Roskal and their Unbeliever friend Sorasta take advantage of the temporary amnesty to flee to a secret Unbeliever hideout in the tunnels beneath the settlement. There, they study mysterious machines which seem to be monitoring the progress of the eruption; perhaps they can also control it...

Peri's fear causes Kamelion to revert to his own shape, and when that proves too difficult to hold, an inchoate, hybrid robot/Howard form. Kamelion gives the TARDIS comparator to Peri, begging her to get it to the Doctor and warn him about the Master, but is overwhelmed once again and fully transforms into the Master. He takes Peri out to assist the real Master, but another quake causes the Master's TARDIS to topple over onto its door. Falling rubble temporarily incapacitates Kamelion, and Peri takes the opportunity to run for it. Kamelion recovers but is unable to lift the Master's TARDIS -- and without the comparator he can't use the Doctor's TARDIS to get inside. He chases Peri into the wasteland outside the city to recover the comparator.

The Doctor is beginning to realize that Turlough wants to avoid people from his homeworld for some reason, although Turlough still wants to look for survivors. They are spotted and captured by Amyand, but once it's determined that they are not from Logar they are warmly welcomed into the Unbelievers' hideout. There, Turlough is surprised to find that they're natives of Sarn, not Trion colonists. He identifies the machines in their hideout as Trion monitoring equipment and geothermal energy taps, capable of re-routing the flow of volcanic gases within the settlement but useless for controlling the eruption. In fact, as the Doctor informs the startled Unbelievers, the tunnels which have been their refuge are volcanic vents which will soon fill with molten lava. He offers to take them to safety in his TARDIS, and Amyand realizes that the Doctor is the "Outsider" whose arrival was reported earlier -- proof positive that he isn't a messenger from Logar after all.

Peri, searching for the Doctor and Turlough, is confronted by Kamelion, and tries to hold him back by threatening to destroy the comparator. He points out that if she does so she will be stranded on Sarn forever, but she breaks the stalemate by calling to Kamelion to show his true face. Her will temporarily overwhelms the Master's, and she flees as Kamelion reverts to his hybrid state. Moments later, Timanov and the lookout arrive -- and the Master has Kamelion draw on their awe at his shining silver appearance and reshape himself into the Master's form once again. Timanov's faith has been fulfilled; the Outsider has come...

The Unbelievers return to the meeting hall, where Amyand makes political capital of the strangers' appearance. The Doctor and Turlough note that the holy artefacts scattered throughout the hall are relics from a Trion ship, including a transceiver which they might be able to use to call for help from Trion. But Turlough seems strangely reluctant to do so. When he learns that Malkon was found in the forbidden lands with the mark of Logar on his arm, he urges the Chosen One to lead him there, telling the Doctor that he thinks these relics came from his father's ship. Malkon is initially reluctant to enter the forbidden lands, until Turlough reveals the "mark of Logar" on his own arm... which means Malkon must also be from Trion. Malkon leads him to the forbidden lands to find the truth, and they find the wreck of a Trion spaceship. There, Peri stumbles across them and is relieved to see a familiar face at last. She tells Turlough her story, and he's horrified to learn that Kamelion is once again under the influence of the Master.

Turlough, Malkon and Peri return to the settlement, and Turlough sends Malkon to save the Doctor and warn him about Kamelion while he and Peri go to the Unbelievers' hideout to re-route the gas flow away from the sacrificial chamber. Malkon tries to stop the sacrifices, and although the Master loses some ground when he doesn't immediately recognize the Chosen One of Logar, he regains it by threatening to withhold the gifts of Logar. Malkon tries to intervene and is shot by a trigger-happy Elder, and moments later Turlough succeeds in diverting the gas flow, causing the fire to go out. Timanov concludes that Logar, angry that his Chosen One has been struck down, has refused the sacrifices.

Malkon manages to warn the Doctor about Kamelion before passing out, and the Doctor tries to use his willpower to overcome the Master's influence. But Kamelion's reversion to his shining hybrid form is seen by Timanov as a sign from Logar. The Master, through Kamelion, orders Timanov to place the Doctor and his friends in the sacrificial chamber, and the Doctor, busy concentrating on Kamelion, doesn't offer any resistance... only to realize too late that some property of the chamber blocks his psychic wavelengths. The Master thus regains total control of Kamelion.

Peri gives the comparator to Turlough and returns to the meeting hall to lend her support, but finds she is no longer able to influence Kamelion. Kamelion, sensing that Timanov is growing uneasy due to the threats of death and the lack of promised gifts, decides simply to leave the Doctor sealed up to die in the imminent eruption. He informs Timanov that the gifts are buried safely at the ruin and leads the Sarns to them, taking Peri along as an unwilling assistant. The Doctor, meanwhile, discovers that the chamber walls are caked with volcanic residue and wonders what properties blocked his attempt to control Kamelion.

Once the others have gone, Turlough enters the meeting hall and releases the Doctor and the Unbelievers. He is horrified to see that Malkon has been shot and vows revenge, telling the Doctor that he thinks Malkon is his brother. Malkon is taken back to the Unbelievers' hideout to rest, while Amyand leads the Doctor and Turlough to the ruins to stop the Master. The question still remains -- why is the Master going to such lengths to recover Kamelion? What does he need him for?

Under Kamelion's supervision, Timanov and the Sarns dig up the Master's TARDIS, and the Doctor, Turlough and Amyand arrive just in time to see Kamelion drag Peri into the ship with him. Turlough reveals the mark on his arm and informs the shocked Timanov that he has been Chosen by Logar to replace Malkon. While Turlough berates the Elders for following the false Outsider, the Doctor and Amyand enter the Doctor's TARDIS -- only to find that Kamelion has removed the temporal limiter. The Doctor is thus unable to prevent the Master from escaping, but thanks to Kamelion the two TARDISes are now operating in parallel -- and to the Doctor's surprise he finds that the Master has remained on Sarn...

Kamelion forces Peri to carry a small black box into the console room, and Peri realizes it must be his control box -- which he doesn't quite deny. They have materialized inside a seismic control chamber in the heart of the volcano, and when Peri seems disinclined to help him Kamelion uses the Tissue Compression Eliminator on two thermal suits to demonstrate what will happen if she disobeys. He triggers a minor quake at the ruins to amuse himself, and then sets about rerouting the gas flow to tap the pockets of numismaton gas beneath the planet's surface.

The Doctor compares notes with Amyand about Logar, and when Turlough leads the Sarns into the TARDIS to escape the quake the Doctor is able to bring up a picture of the god on the console -- although to his and Turlough's eyes it looks like a man in a thermal suit. Timanov admits that as a young man he saw the fire god himself and was encouraged to inhale the gases from the heart of the volcano, after which he returned to the settlement feeling stronger and fitter. Perhaps that's what the Master needs as well. The earth tremors die down and a blue flame burns from the volcano's crater, signalling a time of great favour from Logar. As the Sarns prepare to bring their sick and injured to the meeting hall, the Doctor demands answers from Turlough -- if this colony was abandoned, why was his father coming here and why did the Trions continue to maintain the volcanic control system? Turlough is still reluctant to explain, despite the Doctor's threat to end their friendship if his silence abets the Master's crimes. The Doctor returns to the meeting hall to analyse the deposits in the sacrificial chamber, and determines the presence of numismaton; but why does the Master need such quantities? In any case, he can use it to save Malkon's life if he can organize the gas flow.

Peri recoils in shock and knocks over the box, sending the tiny Master tumbling to the floor and breaking his control over Kamelion. She soon recovers and pursues the Master with a shoe, but he takes shelter inside the TARDIS console. He short-circuits the door controls, unaware that Kamelion has reverted to hybrid form and is unable to help him. Peri takes the opportunity to flee and finds a way back out to the surface.

The Doctor reroutes the numismaton gas flow into the sacrificial chamber, and Turlough carries Malkon inside, saving his life. Timanov hands over the "gifts of Logar" to the new Chosen One, and Turlough recognizes one of the "gifts" as an electronic key from his father's ship. He admits to the Doctor that he can use it to summon help from Trion and save the Sarns, but if he does so he'll betray his location to his enemies. He was sentenced to exile on Earth following a civil war on Trion; his father and brother were exiled to Sarn, which had become a prison planet for "undesirables". By now the Custodians will know that Turlough has escaped, but he nevertheless agrees to face his responsibilities and call on his homeworld for help.

Turlough and Roskal set off for the crash site while Amyand guides the Doctor up the slopes of the volcano, searching for a way into the control centre. Amyand is furious to realize that the worship of Logar was inspired by sightings of Trion vulcanologists, but the Doctor points out that thanks to him, his people are now shaking off the shackles of their superstitious beliefs. The volcano begins to erupt, and flowing lava traps the Doctor and Amyand on the mountain; fortunately, Peri finds them and leads them back to seismic control. There, the Doctor is able to temporarily stabilise the eruption, although it's too late to prevent it completely; the Master has triggered a surge of numismaton gas which will soon reach the surface of the planet and completely disrupt the stability of the Trion control systems.

The Doctor, Amyand and Peri enter the Master's TARDIS, where the Doctor removes its temporal stabiliser for use in his own. He then confronts the shrunken Master, who admits that he made a slight error while trying to increase the range and power of the TCE; but soon the numismaton surge will restore him to full health and size. Kamelion revives, transforms into the Master and forces the Doctor, Amyand and Peri out of the Master's TARDIS while he pilots it into the centre of the gas surge. The way out is now blocked by a sheet of flame, but there remains one full-sized thermal suit; the Doctor gives it to Amyand and tells him to take the temporal stabiliser to Turlough.

Turlough uses the transmitter on his father's ship to contact Trion for help, thus giving himself away. He and Roskal return to the settlement and urge the Sarns to evacuate to the ruins, where the Trion ship will soon land. Timanov refuses to go until Logar himself walks into the settlement... only to remove his helmet and reveal himself to be Amyand in a thermal suit. Amyand gives Turlough the temporal stabiliser and invites Timanov to join the evacuation, but despite everything Timanov's faith in Logar remains unshakeable, and he remains in the meeting hall to face death with dignity. Turlough repairs the TARDIS and programmes it to dematerialize automatically; as it has been remote-paralleled to the Master's TARDIS it will rematerialize in the seismic control chamber. He then departs the TARDIS to face the Trion captain, Lomand...

The Doctor rigs up a makeshift device which will jam Kamelion's circuits. Kamelion pilots the Master's TARDIS into the centre of the gas surge and carries the shrunken laboratory out of the ship. He then approaches the Doctor, preparing to shoot him -- but as soon as he's close enough, Peri triggers the Doctor's weapon and gives Kamelion the equivalent of an electronic heart attack. Kamelion apologises for his failure and begs the Doctor to kill him, which he reluctantly does. The numismaton surge hits just as the Doctor's TARDIS materializes, and the Doctor sends Peri to safety as the Master emerges from the box, growing to full size and strength within seconds. But the Doctor injects calorific gas into the surge, and the Master burns in agony and is apparently vaporised by the incredible heat. The emotionally shaken Doctor returns to his TARDIS and departs as the surge overwhelms the control systems and the chamber starts to fall apart.