A space shuttle threads its way through the asteroid belt. Three young men in silver space suits ride within. Meeker, at the controls of the craft, is forced to switch to manual to evade the hurtling rocks. When he drifts off course and tries to switch back to automatic, the computer is strangely unresponsive. Safran, the commander, takes over the controls in frustration.
The shuttle drifts into a strange mist as Meeker laments with Silvey the boring posting awaiting them on Titan. As they talk, the shuttle comes to the center of a broiling gas. The computer notes an unidentified organism in their path. Lightning bolts flash through space, striking the shuttle. But it then informs the confused crew that "contact has been made."
The TARDIS drifts through space. A tall man with curly brown hair and a young woman in animals skins wander into a gleaming white control room. Leela has never seen this control room before. The Doctor tells her he had grown bored with the color - it showed a singular lack of imagination.
The TARDIS stops. The Doctor checks the instruments and figures they are somewhere between Jupiter and Saturn. It is about 5000 AD -- well before Leela's time. The date is momentous -- the time when humanity breaks out of the Solar System to conquer the Galaxy.
The shuttle lands normally on Titan. The shuttle crew, now fully covered in space suits, ominously take weapons from a rack and enter the base. The old crew greets them on the PA system, welcoming them warmly to Titan. They are delighted to be finally ending their duty. They relief crew, still wearing space suits, enter the lounge where the departing crew is celebrating. The crew encourage them to get settled in and the trio of relief crew raise their weapons and fire. All three men fall to the ground.
Safran opens his visor to reveal a face covered in strange gray scales. He notices that the station supervisor - Lowe - is not among the bodies. They need to dispose of him before they can breed and multiply.
Lowe is enjoying a celebratory drink in the control room when he sees the carnage on the monitor. He sees Safran on another monitor and asks what has happened. But Safran responds strangely, not seeming to recognize his own name, telling Lowe that the base is most suitable. Lowe, alarmed, sends out a distress signal and begins to don his spacesuit.
Meeker and Silvey quickly break into the command room but Lowe has apparently fled to the surface of the planet. They close the airlock, content to let him suffocate in the methane atmosphere.
Leela practices writing out her name while the Doctor works to reset his controls. Suddenly, Lowe's distress call sounds from the TARDIS - it has taken half an hour for it to reach them. The Doctor decides to go and help. But the TARDIS is already in danger, the strange gray mist closing in on the time machine. As the Doctor sets the coordinates, Leela frets and tugs at his long multi-colored scarf. She tells him she can feel something evil. A message shortly arrives from Safran telling them to disregard the mayday. But Leela is not convinced - she thinks it is not a human speaking.
The mist closes in on the TARDIS and a purple glow envelopes the Doctor as he bends over the console. He dismisses it as a St. Elmo's fire. But then lightning flashes out toward the Doctor and a gruff voice sounds in his head that "Contact has been made". He collapses to the floor of the console room.
Safran and his men are laying plans for remodeling the Titan base when the voice sounds in their heads, telling them that the nucleus has found a host. His arrival is imminent.
The TARDIS lands on the Titan base. Inside, lightning flashes up into Leela's eyes -- but she is unaffected. She closes the open panel in the console and bends over the unconscious Doctor.
The infected crewmen approach the TARDIS. Safran informs his crew that there is another with the host - but she has been rejected. They must destroy her.
The Doctor wakes up disoriented and confused. He can't quite remember what happened. He opens the door to go out into the base but Leela slams the control down, closing it. She tells him that something evil is out there, waiting for them.
Lowe returns from the outside and takes a weapon from the rack.
The Doctor and Leela continue to argue about going outside. Leela is now claiming that the Doctor has been affected by something.
Lowe closes in on the murderous relief crew and orders them to drop their weapons. They whirl and open fire. Silvey drops dead from Lowe's accurate fire; the others pursue Lowe. He tries to evade them by hiding in a freezer. But they quickly realizes where he is, shut the door and close off the oxygen supply.
The Doctor emerges from the TARDIS to find the base empty. He sounds a loud duck call but there is no response. Leela finds the dead Silvey and the Doctor wonders why the called off the mayday. The young woman tells them are in great danger but the Doctor's curiosity has been roused. He goes to the command room to investigate while Leela sneaks off on her own.
Safran and Meeker go back to their planning. They are interrupted by the Doctor. They recognize him as the Nucleus and tell him they are preparing the hives. He is baffled and tells them he responded to the distress call. The two men turn, revealing their scaly faces. The Doctor is alarmed but, before he can do anything, lightning flashes out of them and into his eyes. He now confidently states that he is the Nucleus -- that the reject will be destroyed and breeding from his nucleus will begin.
Leela finds the freezer and opens it. A half-frozen Lowe tumbles out.
Safran hands the Doctor a blaster and tells him to kill Leela. The Doctor agrees without question.
Leela bring Lowe into the lounge and warms him with some coffee. She explains that they answered his distress call. He can't understand what is happening -- the relief crew have changed somehow and all he knows is that they say that Titan is suitable for their purpose. Leela assures him that the Doctor will know what to do.
The Doctor coldly calls out to Leela. Lowe warns her it could be a trap. The two of them hide as the Doctor tells her that the place is most suitable. In the middle of calling out to her, he begins to break the Nucleus' control. He tells it to leave him - that he can't kill Leela. Meeker assures him that he can.
Lowe knocks a glass off a table. The Doctor turns to the lounge with refocused purpose. Meeker goes in first. He attacks Lowe but Leela throws her knife into his back killing him. As she rushes out, Lowe bends over the dying crewman and asks him what the purpose is. Meeker's only response is a flash of energy that infects Lowe.
Lowe runs past Leela into the base. The Doctor closes in behind her, his hand now covered in silver fur. He raises his blaster and points it at his companion's unprotected back.