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.

The voice in the Doctor's head urges him to kill 'the reject' Leela. But the Time Lord fights against it and is able to alert Leela, who ducks. The shot goes wide, missing the girl. He collapses to the ground, fighting the infection with all he can. It recedes from control, but only for a moment to gather its strength.

Leela asks why she is not infected but the Doctor doesn't know. He then has to lapse into a trance to save his strength.

Lowe finds Safran and tell him the Doctor is resisting his host. They agree for Safran to remain behind while Low pretends to be uninfected. This will allow him guard the Doctor and kill Leela. When she approaches, Safran lays down, pretending to be dead. Lowe puts goggles on, concealing the infection around his eyes. He tells Leela that he killed Safran but was injured in a flash from the blaster.

Leela takes him to the Doctor and asks what they should do. Lowe tells them of the Bi-Al foundation in the asteroid belt, where they could potentially get assistance. The Doctor wakes from his trance long enough to give Leela the coordinates. She pilots the TARDIS to the Foundation, a complex that occupies an entire asteroid.

A group of doctors wheel the Doctor off to section X-4 for analysis. The receptionist directs Lowe to the eye station to treat the "blaster flash" and informs Leela that Dr. Marius - the resident expert on extraterrestrial pathological endomorphisms -- will be seeing the Doctor.

Professor Marius, a bearded middle-age eccentric, gets a readout from a small mechanical dog named K-9. He testily informs his assistant that the Doctor is merely in a self-induced coma. But his interest is piqued when K-9 tells him the Doctor is not human. An encephalograph reveals that the Doctor has a noetic virus in the mind-brain interface.

The Doctor sits up and Marius assures him they are hard at work. He tells Marius that the virus seems to feed on his very thoughts -- the trance is the only way to keep it dormant. He realizes that the virus must have attacked the TARDIS first because it was showing the greatest amount of mental activity. He speculates that Leela's instinctive and intuitive way of thinking afforded her some protection. He remembers that the entire Titan crew were infected except for Lowe. But he is then forced to go back into his trance. Marius and Parsons decide to check on Lowe.

A doctor arrives in the eye section to examine Lowe. The man removes his goggles and lightning flashes forth, infecting the physician.

Leela finds Marius's operating room. When she tries to go to the Doctor, K-9 extends his blaster and tells her to stay away. Fortunately, Marius arrives before anything else happens and tells K-9 that she is a friend. He explains to the confused girl that K-9 is his best friend -- a computer he made because he missed his dog back on Earth. He wants to scan Leela to see if he can isolate her immunity factor.

Parsons reports that Lowe and the consultant have disappeared. The men find two more doctors and infect them, growing their army.

Leela is scanned but K-9 can not figure out what makes her immune. Marius decides to operate.

Lowe and his men approach the operating room. They hear the voice of the Nucleus warning them that it is in danger from Marius' investigations.

In deep space, another shuttle is surrounded by the cloud. Shortly after, alarms ring through the Bi-Al foundation, warning that an out-of-control shuttle is approaching. Marius and Parsons have to break off their investigation to give help.

The shuttle crashes into the asteroid with a spectacular explosion. The shock rips through the station, knocking the Doctor off of the table. He climbs back on as K-9 informs them that they are cut off.

Lowe and his men crawl into a service shaft to get the Doctor's level, now confident the can wrest him from Marius.

The Doctor knows that the crash was no accident. Leela realizes they will soon be under assault and goes into the hall to defend the operating room while the Doctor asks K-9 about cloning techniques.

Marius and his assistant dig into the crash site and find the bodies of the crew. They have been infected with the virus. Marius tells them to seal off the area.

Lowe and his men emerge from the service duct into withering fire from Leela. They dive for cover as Marius and Parson return to the operating room.

K-9 tells the Doctor that no clone has ever lasted longer than ten minutes. The Doctor asks Marius to clone him before the virus take over his mind.

Leela retreats to the operating room, telling them she's out of ammunition. Marius gleefully send out K-9 to fight Lowe and his men. K-9 knocks out one with his blaster. Lowe decides to withdraw and try talking to Marius.

Marius takes cells from the Doctor and Leela to make clones. He explains that the technique he uses will give the clones both their memories and their genetic patterns -- but will die after ten minutes. Leela doesn't want to watch, so she joins K-9 in the hallway.

Marius places the cell samples in a chamber as the Doctor's face becomes covered with silver scales. Copies of the Doctor and Leela swell into existence in the chamber. The Doctor clone exits and heads for the TARDIS. When Leela sees him, she asks K-9 to explain, but he can not do so in terms she can follow.

Lowe and his men infects more medics. The virus continues to struggle with the Doctor, trying to force him to surrender. Marius speculates that he has some kind of nucleus within him. The Doctor retrieves equipment from the TARDIS.

Lowe appears on the monitor. He warns Marius that he will destroy the Foundation unless they turn over the Doctor. He has two minutes to decide.

The Doctor returns carrying the TARDIS' relative dimensional stabilizer -- which can be used to shrink the clones. He tells Marius to scoop them up and inject them into his own body. He is bringing Leela with him because she's a huntress and because she's immune. He tells Marius they will exit through the tear duct.

The Doctor and Leela spin and tumble through the liquid until they finally stop in a dingy tunnel. The Doctor tells Leela they're too small to break the surface tension. As they gather their wits, a nerve signal travels past them to move the Doctor's leg.

Lowe calls Marius again and the Professor relents, telling him to come get the Doctor - he has no further use for him. Lowe asks about Leela and Marius pretends he doesn't know where she is. He then tells her that she and K-9 must hold off the attackers long enough for the Doctor to succeed. She sends K-9 to destroy the lift shaft to force Lowe to come from one direction. Marius then gives Marius a weapon and tells his assistant to kill him if he becomes infected. Marius assures Parsons he will do the same if Parsons is taken over.

The Doctor and Leela reach a glittering white network of tissue. Leela thinks he's lost but the Doctor assure her is going straight to the mind-brain interface. As they walk, they are pursued by a white fuzzy cell.

K-9 returns from destroying the shaft and Leela has him blast down a section of wall to act as a barrier. He informs her that enemies are approaching. Lowe sees her and tells her to bring out the Doctor. She cries back, "Come and get him!" and opens fire.

Eight minutes remain as Marius and staff continue to analyze Leela's blood, searching desperately for the immunity factor. Marius begins to believe it is a psychological immunity.

The Doctor and Leela have found a section of his brain with numerous dangling ganglia. Leela senses danger but the Doctor is too busy admiring his own brain. He explains that this section allow him to tune into the Time Lord intelligence but the ability was taken away when he was expelled.

He spots a blockage. But before he can cut through, the white balls descend on Leela -- they are the Doctor's own immune system attacking her as an infection. He jams too nerves together and the cells leave, convinced his liver is disintegrating.

The Doctor jerks on the exam table as the countdown continues.

In the hallway, K-9 shoots one of the infected men. But before he dies, he managed to infect the robot dog.

The Doctor cuts through the blockage. He know asks Leela to lead -- they will need her skill as a huntress to find the Nucleus.

Lowe orders K-9 to kill Leela. The dog troddles over and guns her down, then shuts himself off. Lowe, pleased, advances on the operating room.

In the Doctor's brain, Leela feels a bump on her head from her real self being knocked out. The Doctor leads her into a dark chasm, telling her it is the gap between the mind and the brain. He follows a narrow bridge into the abyss. Leela reluctantly follow, holding onto his long scarf.

The countdown reaches five minutes. Marius, sensing the end is near, conceals his nurse in the booth. The door opens and Parsons is gunned down. The men infect Marius and he tells them everything about the nanoclones as his nurse sneaks out. Lowe insists they must foil the Doctor's plan.

K-9 wakes Leela and tells her he has regenerated his circuits and cleansed himself of the virus. He tells her that Lowe has the Doctor. The nurse reports that they are cloning Lowe to inject him into the Doctor. K-9 is certain they must not interfere - their only hope is that the Doctor destroys the Nucleus.

The Doctor and Leela emerge into another portion of the brain. They stand in a strong breeze as the Doctor tells Leela they are going into the world of "dreams and fantasy".

The Lowe clone is injected into the Doctor's brain. He races along their path, guided by the Nucleus, which urges him to hurry.

The Doctor and Leela are surrounded by strange sights and images. Leela can sense they are near the nucleus. But she can also sense Lowe behind them.

The Doctor moves into a small chamber and finds a small black blob with a single glistening claw -- the Nucleus of the Swarm! He confronts the virus but it insists it has the right to survive. The Swarm has waited in space for millennia for a suitable host. It will now not only use humans to conquer space but use the Doctor's TARDIS to conquer time.

Leela attacks Lowe. He shoots and wounds her, but she stabs him. She staggers off while the Doctor's phagocytes destroy the Lowe clone.

The Doctor clone is beginning to fade. Leela staggers in. He takes the blaster from her and shoots the virus, which vanishes. But the clones fade out as well, with the Doctor's word "tear duct" echoing through the brain.

The Even as the Nucleus grows to a full-size monster, the silver scales and fur fade from the Doctor's face - he is free of the virus. Lowe and Marius help the monster out of the chamber. It boasts to the Doctor that, thanks to his dimensional stabilizer, it can now occupy the macro-world -- no longer will it be an invisible microbe. The Age of Man has ended -- the Age of the Virus has begun!

The Doctor is not impressed, having heard such boasting before. The Nucleus tells its minions to take him to Titan where he will be consumed by the Swarm.

Leela dons a nurse's uniform and daubs makeup on her eyes to look possessed. She approaches the men and Marius instructs her to push the Doctor's gurney. She frees him instead and the two escape. The Nucleus has to get to Titan, so it instructs Marius to handle the Doctor -- he is to convert the rest of the staff and bring the Doctor to them on Titan.

Leela and the Doctor retreat to the TARDIS. The girl argues that they can still win if they can only get to Titan. But they can not leave without the dimensional stabilizer. The Doctor talks K-9 into knocking out Marius, who is already summoning the remaining staff. The metal dog rolls out, approaches his master and knocks him out with a single blast.

The Titan shuttle blasts off from Bi-Al and speeds it ways to Titan.

The Doctor takes a blood sample from Leela and contrasts it with his own blood to see what they have in common. He quickly identifies the antibody and prepares a solution for Marius and the other infected staff.

The Nucleus is close to spawning time, thrashing back and forth in the shuttle. It urges Lowe to hurry, even if he has to burn out the engines.

As Leela and the Doctor watch, the scales fade from Marius' face. He wakes and asks about Parsons - he can't remember anything. The Doctor explains the situation and urges Marius to copy the antibody so that the human race will be immunized against the virus.

Safran radios Lowe that everything is prepared. The Nucleus continues to urge him on.

Leela is impatient with the slow process of manufacturing the antibody to kill the virus. She suggests they simply blow up the Titan base but the Doctor reminds her they are in a hospital, not an arsenal. Marius gives him the most lethal strain -- if they put it into the spawning tanks, the Virus will die. Leela reminds the Doctor that he doesn't like killing. He tells her that the virus has a perfect right to exist as a virus -- but not as an all-conquering swarm. Leela reiterates her view that they should just blow up the base.

The Nucleus is brought into the Titan base and placed in a cubicle hive to multiply. The Doctor and Leela board the TARDIS. Marius agrees to let them borrow K-9. Lowe arrays his men to defend against any attack. The Nucleus wallows in the cubicle, laying eggs in a massive tank.

The TARDIS materializes in the control room. Safran leaves to warn the others. The Doctor checks the monitor and sees that the Virus is hatching. One of Lowe's men attacks and Leela blasts him to no effect. K-9 finally is able to finish him but at the cost of critically lowering his energy banks. The Doctor realizes they are developing a resistance to radiation. He rushes out, determined to stop the Swarm.

K-9 decoys another guard and the Doctor sends Leela back to the TARDIS. K-9 then distracts Lowe, who blazes away at the dog until it stops. The Doctor tries to get the antidote into the Swarm but Lowe shoots the vials from his hands. Lowe tells the Doctor he has failed and will now be the first to be devoured by the Swarm. He opens the cubicle but K-9 expends his last ounce of energy stunning the base commander and the Doctor shoves him into the cubicle. The Doctor then has to drag the exhausted dog back to the TARDIS. The Nucleus calls out to him from the cubicle as it near its apotheosis.

Leela hides behind a gas tank, ambushes Safran and cuts his throat. When the Doctor returns and tells her he has lost the antidote, she suggests knifing them all - but the Doctor tells her there will be thousands of them. He tells her to take K-9 back to the TARDIS and then spots the tanks of oxygen she was hiding behind. He quickly opens a valve. He rigs a blaster to the door of the cubicle and opens a valve on another tank. As inflammable gas fills the base, the Nucleus taunts him.

The Doctor races into the TARDIS and takes off -- only to realizes he has left K-9 and Leela behind. He returns and they rapidly board.

The Nucleus shoves against the cubicle door and sets off the blaster. The charge ignites the methane and oxygen. The Titan base erupts in a massive explosion, destroying the Swarm.

The Doctor watches delightedly from the TARDIS. He congratulates himself on the good idea but Leela reminds him it was her idea. He says she should be proud. She confusedly replies that she is.

They return to Bi-Al to bid farewell to Marius. The Doctor explains how much of a help K-9 was and the Professor grins that the dog has taken a liking to them.. He has to return to Earth but is unable to take the heavy metal robot. Leela begs the Doctor to let K-9 join them and he agrees.