The Doctor, Sarah and Harry return to Nerva and find a deadly mystery.
The Fourth Doctor, Harry and Sarah Jane use the Time Ring, spinning their way through time and space back to Space Station Nerva. They land in the control room they left when they last beamed down to Earth, but Sarah notices the TARDIS is not there. The Doctor says 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 from Nerva Beacon, which is under plague quarantine. Professor Kellman, a planetary surveyor, asks Commander Stevenson how long they can run a fifty-man station with three men, but the other officer, Lester, thinks they can manage. Nerva is on a thirty-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 travelers 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 solar flares 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 cataloging its rocks. Voga had drifted into the solar system fifty years before and had been captured by Jupiter’s gravity. An asteroid of that size drifting between star systems could not support life, he says, and warns against going down to Voga and spreading the plague. Warner logs the call anyway. The Doctor opens the sealed door, which activates an alarm.
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. 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, leveling 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 halt 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 device. Stevenson tells the Doctor about the asteroid, formerly Neo Phobos, but renamed Voga by Kellman. The Doctor recognizes the name, Voga, the Planet of Gold, and realizes 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 realizes 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 is attacked by the Cybermat.