The TARDIS materializes on Androzani Minor, one of a pair of twin worlds in the Sirius star system. Androzani Major has been industrialised by human settlers, but Minor has largely been ignored due to its harsh environment. The core of the planet consists of heated mud, and when its orbit takes it close to Major, the tidal effects cause boiling mud to flood the planet's surface. The Doctor finds evidence that a ship recently landed and delivered some sort of cargo to the nearby caves (actually mud blowholes). Followed by a reluctant Peri, he sets off to investigate, and on the way he satisfies her curiosity about the celery stalk in his lapel -- it's to detect the presence of certain gases to which he is allergic. As they explore, Peri slips on the polished cave floor and tumbles into a sticky white puffball. The Doctor pulls her out, getting some of the deposit on his own skin in the process. It stings slightly, but doesn't seem to be worth worrying about...

Elsewhere, a human soldier is surprised, attacked and eaten alive by a ferocious monster; in another, less distant cavern, gun-smugglers await the arrival of androids who are to collect the latest shipment. They hear someone approaching and hide, and the Doctor and Peri arrive and discover the weapons. Soldiers then arrive, arrest the Doctor and Peri and take them back to army headquarters for interrogation. They try to defend their innocence to General Chellak and his adjutant Salateen, but their interrogation is interrupted by a call from Trau Morgus, CEO of the Sirius Conglomerate, the corporation which owns Androzani Minor. Until now the war has been a fiasco; the rebel Sharaz Jek has spirited away the planet's entire store of refined spectrox, and his androids have been running rings around the Federation soldiers for months. Morgus demands to see the captured gun-runners, but after seeing the Doctor and Peri he seems to lose all interest in them and orders Chellak to execute them immediately in order to boost morale back home. The Doctor is appalled, but Chellak knows that an appeal would be useless; Morgus' corporation practically owns the government as well. Even though he was beginning to believe that the Doctor and Peri were innocent after all, he prepares to execute them.

Deep within the caves, the masked Sharaz Jek has monitored the broadcast between Morgus and Chellak, and has become captivated by Peri's beauty. As the Doctor and Peri await their execution, Peri succumbs to despair while the Doctor tries to piece together what's happening here. What's so important about spectrox that it's worth a protracted war? Why are they fighting android rebels? Why is Morgus so determined to see them dead? Why was Major Salateen's behaviour so strangely detached? And is the rash that's begun to cover Peri's legs and the Doctor's hands anything to worry about? The soldiers have almost finished work on the execution yard, but as the Doctor and Peri watch them, a secret panel opens in the wall of their cell...

The gun-runners, spurred on by their violent leader Stotz, surprise and gun down the patrol which was escorting the captured weapons back to the army base. The gun-runners then dispose of the weapons before the army can get hold of them and flee before reinforcements arrive.

Upon learning that the Northcawl copper mine is over quota, Morgus sends someone to deal with the matter... privately. His efficient secretary Krau Timmin escorts the President of the Praesidium into Morgus' office, where Morgus supplies him with a very small amount of refined spectrox from his private reserves. Once refined, spectrox extends the human life span; the President, who is over eighty years old, doesn't feel a day over fifty. But the only source is now controlled by Sharaz Jek, and supplies on Major are becoming depleted. The scarcity of spectrox has resulted in record profits for Morgus' company... but the demand for spectrox is becoming strident, and soon the Praesidium may be forced to bow to Sharaz Jek's demands. As Morgus considers this worrying prospect, Krau Timmin announces that the execution is about to take place.

Morgus switches off as the bodies are cut free of the execution posts, dismissing the Doctor and Peri as criminal riff-raff, and suggesting to the President that all people in the cities without valid work permits be sent to the Eastern labour camps to cut down on crime. It occurs to the President as he leaves that Morgus has been closing plants in the West and building them in the East -- which means that if he were to accept Morgus' suggestion, the exact same people would be working for him again, only this time without pay...

Chellak and Salateen discover that they've just solemnly executed two androids. Humiliated, and realizing that his career will be finished if word of this gets out, Chellak decides to cover up the incident and send the only other soldier who knows on a suicide mission. He assumes that the Doctor and Peri were androids all along, but in fact the real Doctor and Peri have been rescued and taken to Sharaz Jek's base to become his companions in exile. The real Major Salateen is also a prisoner in Jek's base; Jek kidnapped him months ago and replaced him with an android double who informs him of every move the army makes. Jek will not be seriously threatened for years, and long before them the people of Major will rise up. And Jek will give them all the spectrox they want -- when he has Morgus' head on a plate.

Back on Major, Morgus is satisfied to hear that the Northcawl copper mine has just exploded, solving the problem of overproduction and causing the market price of copper to increase. Meanwhile, Stotz has to put down a minor rebellion among his fellow gun-runners, some of whom want to cut their losses and go. Stotz nearly kills the mutinous ringleader, Krelper, but lets him live... for the moment. Back at the army camp, the Salateen android reports to Chellak that a mud burst may be imminent.

The real Salateen believes that Jek will kill him now that he has more interesting companions, and thus bursts out laughing when he realizes that the Doctor's and Peri's rash and cramps were caused by exposure to a spectrox nest. The puffball they encountered earlier was a raw deposit left by the bat colonies, and although it's an elixir of life once it's refined, it's a deadly poison in its raw state. The Doctor and Peri are already in the second stage of spectrox toxaemia, and unless they can get the milk of a queen bat, they'll soon be dead. Most of the queens have retreated from the android harvesters into the lower levels of the caves, where there is little oxygen, and some sort of creature lives in the mud of the planet's core and feeds on anything that moves...

Stotz contacts Jek and demands payment, as the shipment was made in good faith and it isn't his fault that the androids never showed up. Before Jek goes Peri makes the mistake of asking why he hates Morgus so much, and Jek flies into a rage. He built the androids to harvest spectrox on the understanding that he and Morgus would share the profits equally, but Morgus supplied Jek with faulty equipment and left him to die in a mud burst. Jek survived but was scalded for life, and blames Morgus for his disfigurement. He departs, leaving androids to guard his prisoners; all of his androids are programmed to shoot on sight any human who isn't wearing one of Jek's protective belt-buckle signallers.

The Doctor decides to risk approaching an android, which is confused by his inhuman double pulmonary system and allows him to get close enough to disable it. Peri takes the one remaining belt-buckle, and Salateen accompanies her and the Doctor as they return to the TARDIS, intending to fetch oxygen masks in order to get to the queen bats and their milk. But in the caves outside they are shot at by a patrolling android and the Doctor is knocked unconscious by a ricochet. Salateen uses Peri as a shield, and the android lowers its weapon upon receiving the recognition signal from her belt-buckle. Salateen then blows the android to bits and drags Peri back to the army base, ignoring the Doctor's calls as he recovers and tries to find Peri again.

Distracted by the gun-runners, the beast turns on them again, and the Doctor flees as the gun-runners are forced to retreat. An amused Jek is waiting for them; now they know the price of betrayal. The Doctor somehow gets turned around in the caves and runs into them again, and Jek tortures him until he admits that Salateen must have taken Peri back to the army base. Desperate to get her back, Jek agrees to let Stotz take the Doctor back to Major for questioning. Stotz believes the Doctor to be a government spy; why else would he be here snooping around? The Doctor is now entering the third stage of spectrox toxaemia -- numbness and muscle spasms -- but Stotz doesn't care; he'll live long enough to be questioned...

Salateen and Peri return to the army base, where Peri collapses while Salateen explains the truth to the increasingly appalled Chellak. They decide to use the Salateen android to feed disinformation to Jek, but unfortunately the android discovers their ruse when it hears Peri's delirious whimpers through the wall. It departs without giving itself away. Peri isn't feeling well at all, but Chellak doesn't care; she and the Doctor have been consorting with the enemy, and that carries the death penalty. As soon as they're ready she'll accompany the first assault on Jek's base. Chellak arranges for more belt-buckles to be manufactured, and Salateen suggests telling Morgus of the planned assault on Jek's HQ and giving false co-ordinates to throw Jek off guard when he taps into the broadcast. The Salateen android is sent on a make-work mission to get it out of the way, but it takes the opportunity to report to Jek -- who realizes that the real Salateen will now be free to walk around the camp, leaving Peri unguarded.

As the gun-runners return to Major, Stotz chains up the Doctor in the bridge and calls his boss -- Morgus. As Stotz reports what happened, putting a spin on the story to make himself sound good, Morgus notices the Doctor in the background. When Stotz erroneously identifies the Doctor as a government spy, Morgus concludes that Chellak faked the execution on orders from a higher authority, which means the President must suspect Morgus' double-dealings. Morgus orders Stotz to remain in orbit while he considers the implications. Stotz, furious, leaves the Doctor chained up and heads off to refresh himself. As soon as the Doctor is alone he starts trying to escape.

Chellak reports on his planned attack to Morgus, and Morgus, after some consideration, calls the President to his office and reports that he has heard rumours of a planned assassination attempt. Shaken, the President agrees to sequester himself and leaves Morgus' office through his private lift -- but the lift door opens on an empty shaft, and Morgus pushes the President in to his death. He then informs Krau Timmin that he intends to travel to Minor personally to negotiate peace, and orders her to have the lift engineer shot.

Jek kidnaps Peri and takes her back to his own base, where he admits that the Doctor has gone. He is now entirely insane and desperately needs Peri's beauty to forget the pain and darkness in the life to which Morgus has reduced him. Peri informs him that the army is planning to attack, but Jek has already seen to that -- he's changed the recognition code for the belt-buckles. Chellak is in for a shock...

The ship's retro-rockets fire at the last moment, throwing off Stotz's aim. The Doctor stumbles out of the ship's airlock and flees across the dunes, pursued by Krelper and another gun-runner. He eventually collapses and falls, but just as they're about to finish him off, a mud burst begins and they flee back to the ship in terror. The Doctor hauls himself to his feet and staggers towards the caves...

Chellak and Salateen lead the assault on Jek's HQ, but only find out that their belt-buckles aren't working when an android patrol guns down Salateen. As a fierce gun battle breaks out, the caves tremble as a preliminary mud burst approaches. The soldiers have no choice but to push forward before the mud burst arrives, and begin to overwhelm Jek's androids through sheer strength of numbers. Jek temporarily leaves his base to repair some of the damaged androids, hoping to buy some time for the mud burst to do his work for him, but Chellak finds him and pursues him back to his base. There, as they struggle, Chellak manages to pull the mask away from Jek's face -- and he's so horrified by the sight beneath that Jek is able to overpower him and push him outside, into the path of the mud burst. Jek seals his base off from the mud burst and returns to Peri -- who screams at the sight of his exposed face, sending Jek into a paroxysm of anguish.

Krelper and his fellow gun-runner return to their spacecraft, where they are surprised to see Stotz talking with Morgus. Morgus is waiting to hear whether the President confided his suspicions to anybody before his death; if so, Morgus will be forced into exile, but he has funds salted away on the outer planets -- and he intends to take Jek's spectrox with him. Stotz has guessed the approximate location and they can use the army's attack as cover for their raid. Morgus calls back to Major to find out what's going on -- and is stunned to learn that Krau Timmin has gone to the Praesidium with all the evidence she's collected of Morgus' dirty dealings. All of his wealth has been confiscated, including the funds he'd salted away on the outer planets. Now Krau Timmin is the CEO of Sirius Conglomerate, and Morgus is just a man on the run. Krelper and his fellow gun-runner decide to cut their losses and depart, but Stotz shoots them both and accompanies Morgus into the caves. But this time Morgus isn't his boss -- they're equal partners. Morgus has no choice but to accept the change in circumstances.

The Doctor evades the mud burst in a sheltered alcove and steps over the bodies of soldiers and androids scattered around Jek's base. Inside, Jek, his mask back in place, is cradling the dying Peri. The Doctor's celery has no restorative effect on Peri's human olfactory system, but Jek, as desperate to save Peri as is the Doctor, gives the Doctor an oxygen mask and directions to the lower levels of the caves. The Doctor negotiates the caves and crevasses -- stumbling across the dead body of the magma creature as he goes -- until he reaches a dormant queen bat, and manages to fill a vial with enough milk to save himself and Peri.

Jek, trying to keep Peri's temperature down, activates a set of extractor fans, but Stotz and Morgus hear the sound and follow it to its source. There, they demand Jek's spectrox, but as soon as Jek sees Morgus he forgets everything else, including Peri, and rips off his mask to show Morgus the horrific disfigurement that resulted from his betrayal. As Morgus stares at Jek in shock, Jek attacks and begins to throttle him. Stotz shoots Jek several times only to be gunned down himself when the Salateen android returns. Jek pushes Morgus' body into the extractor fans, causing them to short-circuit and burst into flame; dying, Jek then staggers into the Salateen android's arms, ordering it to hold him. The android obeys Jek's last instructions to the letter, and doesn't move as the Doctor staggers into the lab, grabs Peri and carries her out. It doesn't even move as the fire spreads, consuming Jek's entire base...