Following on from Genesis of the Daleks, the Fourth Doctor, Harry and Sarah use the Time Ring, spinning their way through time and space back to Space Station Nerva. They land back in the control room they left when they last beamed down to Earth, but Sarah notices the TARDIS is not there. The Doctor tells Sarah that the time ship is drifting back in time towards them and they just need to wait for her to catch up. A door slides open, revealing a dead body, and many more beyond, littering the outer ring of the station.
In a communications room, crewman Warner warns off an approaching spaceship away from Nerva Beacon, which is under quarantine due to a plague. Professor Kellman, a planetary surveyor, asks Commander Stevenson how long they can run a 50-man station with three men, but the other young officer, Lester, thinks they can continue to manage. Nerva is on a 30-year assignment to warn ships away from Voga, the new asteroid it is orbiting, until its presence is updated on all the starcharts of inbound ships.
The time travellers find a sealed door leading to Section Q. The Doctor surmises that this is the same station they left, but thousands of years in the past, before the solar flares that devastated Earth. As the Doctor tries to get through the door, the trio fail to see a silver, snake-like creature — a cybermat — crawling around the bodies behind them.
Somewhere else, an alien tries to contact Nerva, and barely gets through to Warner before he is shot by two more of his own kind. The only place the signal could have come from is Voga, but Kellman tells Warner that he set up the transmat station there and spent six months cataloguing its rocks. Voga had drifted into the solar system 50 years before and had been captured by Jupiter's gravity. An asteroid of that size drifting between star systems could not support life and he warns against going down to Voga and spreading the plague. Warner logs the call anyway. The Doctor, manages to open the sealed door, which activates an alarm, warning the Commander,Lester and Warner of the intruders.
On Voga, Vorus, leader of the Guardians of the mines, orders his men to bury the dead Vogan that was shot earlier. Magrik, his aide, tells him that the dead Vogan was frightened of Vorus's plan. Vorus tells him that they can trust their agent on Nerva. Gold buys humans, and they have more gold on Voga than in the rest of the galaxy. He also points out that The reason the agent had not contacted them is probably because the Cybermen are monitoring transmissions.
In the communications room, the cybermat attacks Warner, biting him before it is thrown off. Warner collapses, glowing veins appearing on his face while Kellman enters and pulls the magnetic log tape from the console. Meanwhile, the Doctor, Harry and Sarah have reached the forward control room, mere seconds before Lester and Stevenson enter levelling their weapons at them. The door behind them slides open to reveal the communications room, and Kellman brings Stevenson to Warner's fallen form.
When Stevenson sees that his crewman has the plague, he prepares to shoot Warner to stop the infection's spread, but the Doctor stops him. The Doctor lies, saying that they are a medical team sent from Earth, and convinces Stevenson to let Harry examine Warner. They take Warner to the crew quarters as Kellman returns to his own room and spies on the Doctor and Stevenson in the communications room using an assembled camera. Stevenson tells the Doctor about the asteroid, formerly named Neo Phobos, but renamed Voga by Kellman. The Doctor recognises the name: Voga, the Planet of Gold, and realises that Cybermen are involved. Stevenson says the Cybermen died out centuries before, but the Doctor points out they merely vanished after attacking Voga at the end of the last Cyber-War. Hearing all this, Kellman contacts a Cybership nearby, its crew commanded by a Cyber Leader with a black helmet. The ship moves towards Nerva.
Warner is dead. When the Doctor examines the body he finds two puncture wounds, indicating that Warner was injected with poison and confirming the Doctor's suspicion that there is no plague. The Doctor says that if he had seen Warner earlier he might have been able to use Nerva's transmat to filter out the poison from his system. The Doctor has another suspicion; investigating Kellman's quarters, he finds the communications device as well as some gold. The Doctor hides when Kellman returns, but Kellman realises that someone has been inside the room. He sabotages the room, electrifying the floor and sending gas pouring up from it. Keeping off the floor, the Doctor reaches the door to open it with his sonic screwdriver. Meanwhile, Sarah, who was watching a sort of futuristic T.V, is attacked by the cybermat.
The Doctor escapes Kellman's room and hears Sarah screaming. He throws the cybermat to the floor and kills it with some gold dust from Kellman's room, but Sarah has already been bitten. The Doctor carries her to the transmat chamber, handing her to Harry, and prepares to beam them down to Voga and back. However, Kellman has taken the transmat's pentalium drive. The Doctor reconfigures the transmat to bypass the sabotaged system while Stevenson and Lester go and confront Kellman. On Voga, Vorus observes a giant rocket, the Sky Striker. He tells Magrik that his agent has informed them that the Cybermen are heading for the beacon. Vorus wants the Sky Striker fitted with its bomb head in four hours.
The Doctor jury rigs the transmat, and Harry and Sarah beam down to Voga. With the poison filtered out, Sarah instantly recovers. As Harry notices that the cavern floor is littered with gold, Vogans arrive and capture them. Harry and Sarah are brought before Vorus, who wants to know who is still alive on Nerva. However, the answers will have to wait. Harry and Sarah are taken away while Vorus answers a call from Tyrum chief Councillor of Voga, who arranges for them to meet.
Lester and Stevenson capture Kellman. The Doctor explains that the Cybermen fear Voga because gold, as a non-corrodible substance, plates their breathing apparatus and suffocates them. The Doctor cannot get Harry and Sarah back without the pentalium drive, but Kellman feigns ignorance, trying to buy time until the Cybermen arrive. The Doctor uses a control box he found in Kellman's room to activate a cybermat, threatening Kellman with it until he reveals that the drive is around his neck.
Harry and Sarah are chained up in a cave, where Harry notes that the chains are solid gold, which is soft metal that perhaps they can file through. Meanwhile, Tyrum tells Vorus that he knows that aliens have come to Voga. He also knows that Vorus wants Voga to emerge as a trading power again and not hide from the Cybermen, who apparently disappeared centuries ago. Because of this, Tyrum no longer trusts Vorus or the Guardians, and will send his Militia to take over the mines. Vorus is furious, but Tyrum says his troops have orders to crush any resistance.
Fighting breaks out in the mines between the Guardians and the Militia. Vorus tells Magrik to keep Tyrum from finding out about the Sky Striker, and to kill the two humans immediately. Harry and Sarah have managed to free themselves, however, and get away before the execution team arrives. They are pursued by more Guardians, who fire at them. Harry and Sarah are cornered and about to be shot when Militia troops appear in the galleries, forcing the Guardians to stand down.
The Doctor has repaired the transmat, but is unable to lock on to Harry and Sarah as they have left the receptor circle. At that point, Lester detects an incoming ship, but it does not respond to their signals. As the Cybership docks, the Doctor recognises it for what it is, but is unable to lock the hatch. The Cybermen come through, impervious to gunfire, and shoot all three men down.
The Cyber Leader tells Kellman that the three men are not dead, merely neutralised, as they are necessary to their plan.
Harry and Sarah meet Tyrum down on Voga and start to explain about their plan to find the TARDIS.
Kellman explains that he set the transmat receptors mere yards from a shaft that leads into the core of Voga. As the environment is hostile to Cybermen, the three men will carry explosives down to Voga and destroy the asteroid. Kellman insists on going down to Voga first to check that the transmat is functioning properly and the Cyber Leader beams him down. There, he runs into some Militia. Not realising the distinction between them and the Guardians, he demands to see Vorus and is taken away while trying to warn them that they are all in danger. Meanwhile, when Harry and Sarah mention Cybermats, Tyrum decides to take them to the gold mines to confront Vorus.
The Doctor wonders what Kellman's reward is, if it is not Voga's gold. He taunts the Cyber Leader, saying that the Cybermen were finished once humans discovered their weakness to gold and ended the Cyber-Wars. Cyber Leader tells the Doctor that is the reason why Voga must be destroyed before the Cybermen begin their campaign again. The Cyber Leader says that Kellman was promised the rule of the solar system after the Cybermen had conquered it.
With Cyberbombs strapped to their backs, the Doctor, Lester and Stevenson are briefed. They are to plant the bombs in the core of the planet, after which they have 14 minutes to return and escape via transmat. If they try to remove their harnesses before they reach the target zone, a secondary explosion will kill them. Their progress will be followed by radar, so if they go off course the Cybermen can use manual controls to explode the bombs. The three beam down, accompanied by two Cybermen, one of them holding a timer. Militia arrive and start to fire on the Cybermen. The Doctor, Lester and Stephenson run away as the two Cybermen kill two out of three Vogans, the other running away. The two Cybermen climb under a rock and enter a chamber with a staircase going up the side. About ten Vogans who were stationed on the stairs open fire on the Cybermen but their weapons are useless and they are all killed by the Cybermens lasers.
None of the three men believe that the Cyber Leader will keep his word about letting them escape, but they have to keep moving towards the target zone as they are being monitored. On Nerva, the Cyber Leader declares that Kellman is of no further use to them.
Tyrum questions Kellman, who tells him that he and Vorus were working to lure the Cybermen to the beacon, which Vorus has targeted with a rocket. At that moment, a Militia man arrives to tell Tyrum about the arrival of the Cybermen, and how their weapons are useless. Kellman urges them to use the rocket. Tyrum orders his men to use every weapon they can while he speaks to Vorus. Harry tells Sarah to get back to Nerva and warn the Doctor while he tries to stop the rocket from being fired.
When Tyrum tells Vorus about the Cybermen on Voga, he shows Tyrum the Sky Striker, which he has been working on for two years. However, with the Cybermen already on Voga, they have no time to get it ready. Vorus claims his plans were to just free his people from the fear of the Cybermen and bring them back into the light. Tyrum scoffs, seeing as Vorus has allied himself with Kellman, a double agent and murderer, motivated only by the promise of gold. Harry suggests finding another way into the core to stop the bombs.
The Cybermen continue their slaughter of the Vogans as the bomb timer ticks down even further. Sarah uses a motor boat and transmats back to Nerva, where she overhears the Cybermen monitoring the three men's progress. However, the deeper the three men go, the heavier the concentration of gold interferes with the radar. The men continue onward, however to the centre of the asteroid.
Harry and Kellman, meanwhile, are crawling down a cross shaft towards the same location. With the exit blocked, Harry pushes against the rocks, causing a rock slide. Kellman pushes Harry out of the way, but is crushed to death by a boulder, while on the other side, rocks rain down on the Doctor. Harry exits the shaft and finds the Doctor unconscious. Not realising the danger, Harry tries to unbuckle the Doctor's harness.
Fortunately, Harry is stopped by Lester. The Doctor awakens and conceives a plan. Stevenson will continue on and create a radar trail, while the rest use the cross shaft to surprise and attack the Cybermen with gold. The Doctor and Harry jump the two Cybermen, trying to push gold dust into their chest plates. However, the Cybermen are too strong, and Harry and the Doctor are forced to retreat. Lester leaps onto the Cybermen and undoes his harness, the explosion killing both himself and the Cybermen in a burst of flames.
With the loss of contact, the Cyber Leader orders immediate detonation. Sarah tries to stop them but is thrown to the floor. However, when the button is pressed, no explosion follows. The Doctor has managed to disarm the countdown device, which allows him to release his harness safely. With Sarah tied up, the Cyber Leader now plans to send Nerva, loaded with more Cyberbombs, to crash into Voga to destroy it.
Magrik tells Vorus that the Sky Striker is now ready, but before he can launch it, the Doctor asks them to give him 15 minutes to transmat to Nerva and deal with the Cybermen himself, armed with a bag of gold dust. If he does not contact them by that time, then they can launch the rocket.
The Doctor reaches Nerva and frees Sarah while the Cybermen are loading the bombs. He takes the cybermat and its control box, filling the cybermat with gold dust. The Doctor sends the cybermat to attack a Cyberman, injecting him with the dust and killing him. As Nerva begins to move towards Voga, Vorus sees this and attempts to fire the rocket. Tyrum shoots Vorus, but as the Guardian dies, he triggers the launch.
The Doctor and Sarah's attack on the remaining Cybermen fails; The Doctor is forced by the Cyberleader to tie himself and Sarah up and they are left to perish in the crash. However, the Sky Striker is approaching just as fast. The Doctor manages to untie them both with a trick learned from Harry Houdini, and contacts Voga, instructing them to steer the rocket towards the Cybership that is just leaving. The Sky Striker veers away from Nerva and destroys the Cybership instead. However, the beacon is still on a collision course. The Doctor manages to unlock the gyro controls, skimming Nerva just above Voga's surface until they reach the other side of the asteroid and open space.
The TARDIS materialises in the control room just as Harry arrives via transmat. The Doctor tells his companions to hurry up; he's received a message from Brigadier Lethbridge-Stewart through the space-time telegraph the Doctor left him, which means it is a grave emergency. Although Harry asks if they should say good-bye to the Commander, Sarah tells him not to argue. The three rush into the TARDIS and it dematerialises.