The Doctor, Harry, and Sarah Jane drift through time and space via the Time Ring and arrive back on Nerva. The TARDIS is absent and the Doctor is unconcerned, noting it's drifting back through time. When Harry reaches for the Time Ring, it disappears. The Doctor leaves the transporter room and finds a corpse. Harry determines the man has been dead about a week. Going out into the station, they find numerous other corpses.
In a communication room elsewhere on Nerva, Crewman Warner warns an approaching ship that they have a plague infection and the ship is transferred to new coordinates. The pilot of the ship asks about his brother and Warner asks Commander Stevenson, who tells Warner to lie. A civilian surveyor, Kellman, taunts Stevenson about how he and his two remaining men, Lester and Warner, can't keep things up but Stevenson insists they can and must do it for the next 30 years until the end of their assignment – to warn ships away from a drifting asteroid, Voga.
The Doctor, Harry, and Sarah Jane make their way to a sealed door leading into Q Section. The Doctor determines they are thousands of years in the past from the solar flares then goes to work on the door. They are unaware that behind them a small metal slug-like creature is crawling along the floor.
Warner gets a scrambled signal but it's cut off – he's unaware it's from Voga. The native trying to send the signal is killed by two of his kind. Kellman arrives as Warner concludes the signal could only have come from Voga, but Kellman tells him nothing could have lived and he was on the asteroid for six months and found nothing. Warner logs the call over Kellman's minor objections.
The Doctor burns through the door and reaches through to open the locking mechanism, and almost loses his arm before they get through. Their entrance sets off an alarm and Stevenson and Lester go to investigate.
On Voga, the guards drop the dead man's body in the throne room and their Leader, Vorus, has the body disposed of. Vorus' assistant Magrik warns the dead Vogan was afraid of Vorus' plan and is concerned that their human agent might fail them. Vorus knows of the Cybermen and reassures the frightened Magrik he will eliminate all of their ancient fears.
The silver creature makes its way into the communications room and attacks Warner, latching onto his neck. He manages to throw it clear but quickly collapses. Stevenson and Lester get to the door. Warner's veins start to glow red and Kellman enters, but he ignores the technician and destroys the tape log. The Doctor and the others get to the forward control room and Stevenson and Lester get them at gunpoint.
Kellman shows them Warner but the Doctor intervenes and tells them they're doctors here to deal with the plague. Kellman tries to get Stevenson to kill them but the Doctor points out Warner is infected but they're not, and it may not be a plague. The Doctor and Harry tend to the dying Warner while Kellman goes to his quarters and uses a concealed spy device to listen in on Stevenson and the Doctor. The Doctor notices scratches on the floor and the shredded magnetic tape. He also asks about the new satellite and recognizes the name Voga as the Planet of Gold. He connects the name to the Cybermen, which Stevenson says died out thousands of years ago. Kellman uses a radio device to contact a nearby spaceship belonging to the Cybermen. The Cyberleader orders his crew to set course for Nerva.
The Doctor goes to examine, Warner who has passed away. He finds the injection marks and concludes the transmitter could be used to separate out the poison, but it was too late for Warner. Moving through the corridors, the Doctor spots Kellman leaving his quarter and sneaks in to check it out. He finds Kellman's communication and spy devices as well as a sack filled with gold. Kellman returns and the Doctor hides, but Kellman notices the spy device is askew. He electrifies the floor and then sets the vent to pump in gas.
While Sarah watches a recording on solar flares, the Doctor moves across the furniture and tries to open the locked door with his sonic screwdriver. The silver slug-creature advances on Sarah and then leaps to the attack.