The Krotons was the fourth story of Season 6 of Doctor Who. It was the first story penned by future script editor Robert Holmes, whose prolific contributions to the series would create or contribute to many key elements of the program’s mythology.
On the planet of the Gonds, a ceremony is underway. The two brightest students, Abu and Vana, have been selected to become “companions” of the Gonds’ unseen masters, the Krotons. As Abu eagerly dons his selection robes and enters the Krotons’ machine, Vana’s lover Thara desperately begs her not to go. Those companions who have gone before have never been seen nor heard from again. Gond leader Selris, Thara’s father, is resolute, stating that Kroton law dictates that she must.
The Doctor’s TARDIS lands in a desolate area nearby. Jamie immediately notes the strong odor of sulfur. Zoe suggests that they depart, but the Doctor opts to explore. From a clifftop they see the Gond city below. They encounter a metallic crystalline structure with a hatchway similar to the one in the city. The Doctor is cautious. He identifies the structure as a machine and they hide as a dazed and weary Abu emerges from the hatchway. Before their horrified eyes, he is evaporated by blasts of energy.
Thara and Selris argue over Vana’s selection. The Doctor, Jamie and Zoe enter the city. They are regarded with suspicion; the Gonds have never met strangers before and the area where they landed, the Wasteland, is believed uninhabitable. As a particularly hotheaded young man fights with Jamie, Vana rushes into the Kroton machine. The Doctor’s description of the death of Abu shocks the crowd. The Doctor, Jamie and Zoe rush back to the Wasteland to rescue Vana, followed by Thara. They block the energy beams with rocks and pull her away as she stumbles from the machine. She is near-catatonic.
Back in the city, the Doctor tends to Vana. Selris tries to make sense of what is happening. The Krotons, he says, have long been the Gonds’ benefactors, providing them with knowledge. The Kroton machine landed on the planet thousands of years earlier and the ensuing war nearly annihilated the Gonds and ruined the landscape. Since then, the Krotons have never left their machine. They created the teaching machines to train the Gonds and the laws that require their top students to become companions; the Doctor notes this is a form of self-perpetuating slavery.
Thara and a band of students sneak into the Learning Hall and vandalize the Kroton teaching machines to force them to emerge. They are stopped by a bellowing, mechanical voice ordering them to leave the hall. As the Doctor and Selris reason with the vandals, a portal opens and a metallic, snake-like object emerges and homes in on the Doctor.
The device stops when the Doctor shields his face. Zoe notes that the device must use pattern recognition. This is confirmed when the Doctor deliberately lowers his hands momentarily and it comes back to life. A student attacks the device, which vaporizes him and then retracts into the Kroton machine. The Doctor notes the apparently limited logic of the device’s programming; it seems to think it has completed its task of destroying him. The bellowing mechanical voice returns, ordering them to leave the hall. They obey.
The Doctor and Zoe note the large gaps in the Gonds’ knowledge. The Krotons apparently are training the Gonds for a specific purpose. They leave Jamie to care for Vana and enter the Learning Hall to investigate the teaching machines. On the way, the Doctor investigates a closed-off chamber running under the Kroton machine. He finds the machine’s foundation resembles a root structure — could the Kroton machine be organic? When he returns, he is horrified to see Zoe operating a learning machine. She earns a score twice as high as any Gond, to Selris’s amazement, and is almost immediately summoned to be a Kroton companion. The Doctor takes the test himself; not performing as well as Zoe (scoring higher but requiring more questions), but nonetheless well enough to also be selected quickly.
They enter the Kroton machine and find the interior chamber empty, save for two chairs. They sit and are subjected to a device which activates a pair of tanks filled with a crystalline slurry. Zoe notes that the device converts mental power into energy. They see the tanks and realize that whereas the Gonds’ mental energy was insufficient, their mental energy set the tanks into operation. Two Krotons, crystalline bipeds, form from the slurry and awaken.
The Doctor and Zoe escape from the machine’s rear exit, eluding the blasters, while Jamie breaks in through the front. The Krotons note that the Doctor and Zoe are not Gonds and resolve to recapture them. Jamie, whom the Krotons believe is not a Gond, is captured and subjected to the mental probe. They note that he is not a “high-brain”; consequently the power of the mental probe will destroy him.
One of the Krotons determines that Jamie may be able to provide information about the Doctor and Zoe and turns off the mind probe. They track the Doctor and Zoe as they head toward the TARDIS, and interrogate Jamie as he awakens, learning that they are travelers in space and time.
A group of Gonds, led by Security chief Eelek, approaches scientist Beta about how to attack the Krotons now that the Doctor and Zoe are presumed dead. Beta is reluctant, not out of fear as Eelek infers, but out of a lack of knowledge. The Krotons have taught the Gonds only what they want for them to know. Beta bemoans the inability of the Gonds to think for themselves. Eelek is impatient to act against the Krotons. He also covets Selris’s position.
A Kroton leaves the machine, which they call the Dynatrope, to destroy the TARDIS, intending to trap the Doctor and Zoe on the planet. The other Kroton offers directions to its blind companion.
Thara is also resentful of Selris’s apparent inaction. Although Selris considers the Krotons enemies, he is more cautious about how to fight back than Eelek. He doubts Eelek’s intentions and sees his ambition as more of a power grab. At the council meeting, he is alarmed to discover that in his absence they voted to elect Eelek leader. He reminds Eelek they still have no more weapons to fight back with than they did when the Krotons first arrived and the wasteland is still desolate thousands of years later.
Meanwhile the Doctor and Zoe have retrieved items from the TARDIS and collected sulphurous rocks from the Wasteland. The Kroton captures them, but when Jamie attacks the other Kroton, it is disoriented and they escape. The other Kroton must ignore Jamie to provide directional data and instructs the Kroton to destroy the TARDIS. The Kroton fires a dispersion jet. Zoe is horrified when the mist clears and the TARDIS is gone, but soon after the Kroton returns to the Dynatrope, the TARDIS re-materializes. The Doctor turned on the HADS (Hostile Action Displacement System) before leaving, and they return to the Gond city.
Eelek and his men prepare to attack the Dynatrope directly. Selris and Beta dismiss it as suicide. Selris has his own plan, which he shares with Beta, to attack the support pillars under the Dynatrope (accessible via the closed chamber in the Learning Hall). He hopes the attack will force the Krotons to emerge so they can be attacked directly.
The Doctor and Zoe return to find Thara with the recovered Vana, preparing to leave the evacuated city. The Doctor is alarmed when Thara informs him of Selris’s plan. He hurries off to the learning hall, stopping off at Beta’s lab to leave the sulphur and instructions. His alarm heightens when he realizes Jamie may still be in the Dynatrope. The Doctor enters the underchamber yelling for them to stop. Their efforts cause a massive rockfall.
The Doctor is shaken but otherwise unhurt, though several Gonds are injured, including Thara. The Krotons struggle to stabilize the Dynatrope, which is damaged slightly, while Jamie attempts to escape. Vana provides the Doctor with a small sample of Beta’s sulfuric acid. The Doctor is horrified to learn that Jamie is indeed in the Dynatrope, though he escapes by his own ingenuity, to the Doctor and Zoe’s delight. Jamie assists Beta with making sulfuric acid.
While Eelek and Selris argue over leadership, a Kroton emerges from the Dynatrope, demanding the two high-brains (the Doctor and Zoe) be returned. Their minds are needed to produce energy to repair the drive mechanism and the Dynatrope is quickly running out of energy. The opportunistic Eelek realizes that if they are captured, the Krotons will leave, though Selris suspects that the Krotons will destroy the Gonds regardless. Eelek and his men capture the Doctor and Zoe and force them into the Dynatrope. Selris takes a bottle of Beta’s acid and leaps through the closing hatchway, passing it to the Doctor before being blasted out of existence by a Kroton.
The Doctor realises that if the Dynatrope’s energy is exhausted, the resulting explosion will destroy the city as well. While he stalls them, Zoe dumps the acid into the crystal slurry tank, poisoning the Krotons. Meanwhile, Beta and Jamie reach the clifftop and pour the vat of acid over the Dynatrope, dissolving it. The Gonds rejoice at their freedom. Thara intends to fill his father’s hereditary post as leader instead of the disgraced Eelek. Beta anticipates learning more from the Doctor. However, he finds that the travelers have left.
In the Wasteland, the TARDIS dematerializes.