The Doctor and Jamie lead a bewildered Victoria into the TARDIS console room. As many before and after, she is stunned by the hugeness of the ship's interior. The Doctor tells her the TARDIS is his home as he busies himself at the controls. He explains that the controls guide their flight through time and space. It is a "rather special version" of the time travel technology her father and Maxtible were working on. Still concerned about the nature of time travel, Victoria tries to work out how old the Doctor is. He concludes that - in Earth terms - he must be about 450. She looks askance and even Jamie is concerned at this. An embarrassed Doctor changes subject, pointing out that Victoria's large skirts might be impractical for adventuring.
Jamie leads Victoria off to choose a new wardrobe, admonishing the Doctor to try for a smooth takeoff so as not to scare Victoria. Somewhat stung, the Doctor activates the controls and the TARDIS dematerialises from Skaro.
Far out in the universe, on the barren rocky surface of a planet called Telos, an archaeological expedition is in progress. A group of scientists and astronauts are preparing to blast away at the side of a mountain. However, they are delayed when a member of their party - a soft-spoken giant of a man called Toberman - is not in position. In fact, he is still on the mountain. Professor Parry, the leader of the expedition, shouts him down impatiently. Toberman returns to the side of his mistress, an exotic woman called Kaftan.
All of the scientists and astronauts seem on edge. Clearly much is riding on this expedition and much danger attends it. Captain Hopper, a no-nonsense man in charge of the flight crew, thinks it all daft and is only doing his job. Parry believes that they have located the entrance to the "city of Telos" behind the rockface. Shortly, the blast echoes through the rarefied atmosphere and the rock is blown away. However, it is only after the dust settles that the group can see their objective - a pair of giant metallic doors set into the mountainside. The group whoops and cheers and races up the mountain.
They stare in awe at the giant doors for a moment. They seem somehow oblivious to the two images adorning the walls on either side of the door - crude renderings of what are clearly Cybermen in shining metal plate. Kaftan is first to speak, offering 50 pounds to the first person to open the doors. Parry starts to protest at this latest usurpation of his authority, but one of the astronauts steps forward to claim the money.
The man grabs first one of the handles - several inches above his head - and then the other. The crackle of electricity fills the air and the man jerks and falls. He is dead. The group is stunned, unsure what happened to him. Suddenly another noise echoes around the mountain. It is a laboured engine noise, rising and falling. Something has arrived just beyond their sight around a large outcrop of rock. Unbeknownst to them, it is the TARDIS.
Captain Hopper and his remaining crew - Callum and Rogers - along with scientist Haydon draw their guns and move forward. They are first to catch sight of the unlikely trio of newcomers as they circle the rock. Hopper addresses them at gunpoint. With no other choice, they come forward and join the expedition at the doors.
Jamie is incensed at the reception and is belligerent, but the Doctor moves off in a different direction completely. He spots the dead man and moves to examine him. Suspicion is rampant in the group. A cool-looking middle-aged man called Eric Klieg accuses the Doctor of having some part in the crewman's death, but the Doctor quickly refutes this. He sizes up the situation quickly, blaming an electrical booby trap in the doors.
Parry's assistant, Viner, believes the Doctor and his companions to be a rival expedition to Telos, searching for the same thing. The Doctor will neither confirm nor deny this and demands to know what Parry and his group are searching for. Parry and Viner explain they seek the last remains of the Cybermen. They died out centuries ago for unknown reasons, but it is believed they retreated to Telos, their home, to die. The Doctor takes this all in.
Meanwhile, Hopper is convinced that the expedition is over. One of his men has died and there is not enough money in the world to make the job worthwhile to him now. Parry seems to agree. Hopper and his men bear their crewmate back to their rocket where they will await the scientists as they prepare to leave.
But the Doctor is now resolved to continue the expedition and enter the city, despite Klieg's insistence that the Doctor and his "colleagues" leave at once. But the Doctor cannot leave, now that "that name" has been uttered. Cybermen. His determination shows in his eyes and his voice. Besides, as Jamie points out, they have as much right to be here
as the others. Parry agrees, despite Klieg's continued protests.
There is clearly a strong antagonism between Parry and Klieg. Klieg and Kaftan are here "on sufferance only". They are not archaeologists and despite the fact that Kaftan's money is financing the expedition, she and Klieg have no say in the running of it. The conflict threatens to escalate between Parry and Klieg but Kaftan intervenes - smooth as silk - and defuses the situation. Parry is in charge.
The Doctor has observed this closely and now decides to go into action. He approaches the giant doors, preparing to open them. He attaches a small electronic device to the door and observes its reading. Then he impulsively grabs one of the handles. A gasp echoes round the group. They are certain he is going to electrocute himself. He seems pleased with the reaction, assuring the others the doors are perfectly safe. As he grabs the other handle, nothing happens and all are surprised. But neither do the heavy metal doors open. It is beyond the Doctor's strength to open them.
Jamie moves forward to give it a try, heaving with all his might. But he too is not strong enough. He blames a lack of exercise!
The Doctor, however, has found the most suitable candidate - Kaftan's servant Toberman. He is tall and muscular and perfectly fit. Kaftan opposes putting him in danger but Parry believes this is just the sort of contingency for which he was brought. The Doctor puts the icing on the cake by suggesting Toberman might be scared. The giant moves quickly forward, brushing past the Doctor on his way to the door.
It takes all of Toberman's strength, but the doors at last open and the way is clear to the gloomy interior. Klieg is all ready to enter, but the Doctor urges caution. The electricity may be off but there is still much to worry about. Klieg doesn't care and starts to lead the way in, only remembering at the last minute to let Parry - the leader - go first.
The Doctor wishes to follow the group, but Victoria is frightened and hangs back. The Doctor tries to distract her by complementing her new dress, but she thinks it is too short. He makes her laugh by pointing out Jamie's hemline and her tension eases. Victoria - still reluctant - eventually goes along with the Doctor and Jamie.
The group emerges into a control room, large and echoing and very dusty. At its centre is a control panel with numerous levers and labeled with logic symbols. Before the panel is a large clock-like dial with more symbols and an arrow pointer. There is a short set of steps on the other side of the room which lead up to a large hatch. It is firmly closed but clearly leads downward to some lower level. A table with four stools is off to one side. More of the bas-relief Cyberman heads adorn the walls, staring blankly down at the group.
Parry, Viner, and Haydon are quick to examine control panel. It is dark and dead. Klieg, Kaftan, and Toberman stand off to one side, conspiring quietly. Kaftan is worried about the presence of the strangers and orders Klieg to keep an eye on the Doctor. She clearly has a very strong influence over him and he agrees. She will watch Victoria and Toberman will watch Jamie. She must specifically warn Toberman to watch and not harm the lad.
The Doctor and his friends arrive and the Doctor quickly begins his investigations... with the walls. He announces - correcting Professor Parry - that there are two doors hidden in the walls. He also manages to open both doors by operating a "simple O/R gate" with the levers on the control panel. The archaeologists are excited by this development but Klieg wants to know how he figured all this out so quickly. The challenge in his voice is clear. The Doctor credits it to his own special technique: "keeping his eyes open and his mouth shut". Klieg is stung.
Parry moves on, dividing the group to more easily explore the different rooms. He tries to leave the women behind, but Kaftan won't hear of it. She and Victoria, along with Toberman, join Viner. They will explore one of the doorways the Doctor discovered. Jamie and Haydon will explore the other. The Doctor, Klieg, and Parry will stay here. Parry tells them to return before dark so they can return to the ship before the temperature drops.
As the groups leave, Parry turns his attention to the large hatch in the control room and begins looking for a means to open it. This time, the Doctor steps back to let Klieg have a crack at it.
Viner, Kaftan, and Victoria find their passage leading to a bright room full of instruments and wires. On one end is a pallet with a humanoid-shaped impression in the centre. The Cybermen clearly lie in it for some reason and there is a door - now open - which can be enclose it. In the centre of the room is a tall projector with much wiring which points at the pallet. On the other end of the room is the control panel, patterned after the first. As the group explores, Viner notes the absence of Toberman. Kaftan has sent him back as they "have Viner to look after them".
Viner begins to make notes of everything, but the women are more hands-on in their investigation. The curious Victoria climbs into the pallet while Kaftan tries to make sense of the controls. She deduces that the room is for recharging the Cybermen, the projector used to feed them with a sort of neuro-electric potential. Viner agrees. Victoria ignores Viner's pleas to get out of the pallet but Kaftan appears to welcome the meddling. She seems to be trying to activate the controls in a deliberate sequence as she watches Victoria over her shoulder.
Jamie and Haydon's passage leads to a totally black room with only a small bank of controls in one corner. Jamie listens for only a moment to Haydon's confusing explanation of the rooms alpha meson phosphor lighting system before he is distracted by something on the floor. It appears to be a sort of segmented metal caterpillar with large eyes and antennae. He is amused by it but Haydon is concerned even though the creature appears to be totally dead.
Klieg's preliminary investigations have yielded the rudiments of the logic system that should open the hatch, but the panel is still inactive and the hatch is closed. As the men focus on their task, no one notices Toberman emerge from the passageway and leave the complex via the main entrance.
The Doctor tries to talk Klieg out of continuing, saying that there are some things that should be left undone, even if they can be easily done. This leads to an argument over the difficulty of cracking the logic codes. Klieg is convinced it is very difficult and purposely so to protect the Cybermen's secrets, but the Doctor cannot resist pointing out the pompous mathematician's errors and lack of understanding. Unfortunately, these clues lead Klieg to the answer he has been seeking.
Suddenly the control panel - and indeed the whole complex - comes to life with a burst of power. The panel and the clock-like dial light up and the pointer moves. Klieg is triumphal. A rumbling of energy and machinery can be heard and felt all over. The Doctor is horrified, worrying that perhaps the Cybermen aren't quite as dead as first thought. He hurries off to check whether the others are all right.
In the recharging room, the surge of power is palpable as well. The controls light up and Kaftan reacts quickly. She moves a series of levers which activates the recharging equipment. The lid of the pallet slams shut, trapping Victoria inside before she can even move. Viner suspects this was no accident but worries more about Victoria. Brute force will not open the lid. Kaftan wonders aloud if they are already too late to save the girl.
In the other chamber, Jamie is startled when he sees what he thinks is a movement from the metal caterpillar, but Haydon thinks he's seeing things. He is more interested in the control panel which has suddenly become active. Jamie warns him to leave it be, but Haydon won't listen. At first nothing happens but then the lights in the room dim and a series of pulsating lights appear on one wall. It forms a shifting set of psychedelic patterns and Jamie stares at it as if mesmerised.
Viner has turned his attention to the control panel in an effort to save Victoria but he is too afraid of worsening the situation to touch anything. Logic escapes him in his panic. Kaftan establishes that Victoria is still alive by rapping on the lid of the pallet. She receives repeated raps in response. Relieved, Viner decides to go for help, leaving Kaftan to watch over the situation.
Once she is alone, she decides to finish the job she started. She moves more levers on the control panel and the projector in the centre of the room comes to life and a new hum of power fills the air. It appears that she will succeed in eliminating Victoria, but she is startled from her task by the return of Viner with the Doctor in tow.
Kaftan tries to activate the final sequence while the Doctor's attention is on the pallet, but somehow he sees this and warns her away. He joins her at the control panel and begins to reverse the logic sequence, Victoria's safety uppermost in his mind.
Haydon, too, is transfixed by the pulsating shapes on the wall, but with sheer force of will he breaks the spell and regains himself. He tries to pull Jamie away but cannot. The lad moves as if hypnotised toward the wall, almost into the shapes. Haydon manages to deactivate the controls and the shapes fade and the lights return to normal. Jamie regains himself, almost oblivious to what has happened.
Both men think it unlikely the Cybermen would need a hypnotising machine. Haydon believes the shapes are a target of sorts, with a subliminal centre that soldiers would be trained to see in the shapes. He has heard of something similar before and wishes to explore it further. Haydon instructs Jamie in the control sequence to use and he himself will watch the "target" when it appears.
The Doctor believes he has found the correct sequence of controls to release Victoria and he instructs Viner to stand by at the pallet. Kaftan he only wants to stand back. He is very aware of her part in this "accident". The Doctor is successful as the pallet lid swings open and the projector shuts down. He joins Viner and helps out a sobbing and gasping Victoria. The terrified girl clings to him.
The Doctor warns her to be more careful in future, but his comments are directed toward Kaftan as well. He bustles Victoria out, planning to go and check on Jamie next.
Meanwhile, Klieg and Parry continue their efforts to open the large circular hatch in the control room, unaware of the travails of the rest of their group. The tombs of the Cybermen must be below and the expedition will be a failure if they cannot be explored. Klieg blames the Doctor for his continued inability to open the hatch, convinced he knows more than he's said. This makes Klieg even more determined to succeed on his own. There also seems to be far more than professional interest in Klieg's determination.
The shapes are now back on the wall of the darkened room and Haydon is watching them. But it seems inconclusive. There must be more to the sequence, Haydon reasons, and at his urging Jamie activates the last control - a large button.
The lights change again and a machine noise fills the room. An unseen set of doors opens along the wall with the "target" on it. The Doctor, Victoria, and Viner enter and take in the scene. The Doctor is horrified by this meddling and tires to find out what Jamie has done. The confusion in the room grows. The Doctor hurries to the controls, working with growing desperation to try and undo the control sequence. But he is too late.
From the new doors, a Cyberman emerges, weapon in hand. There is a blinding flash of energy and Haydon collapses with a scream, smoke pouring from his body.