Having travelled into another Universe only to lose the TARDIS in what appeared to be a deliberately engineered environment, the Doctor and Charley now find their progress thwarted by some form of inter-zone resistance. To Charley, this force takes the form of a swarm of deadly insects. The Doctor drives off the swarm with a warrior cry useful for dispersing spirit attacks, and he and Charley catch a glimpse of a desert environment on the other side of the barrier -- but as they try to press through, the resistance returns, stronger than before. Charley then hears a voice in her head which identifies itself as the Kro’ka, the inter-zone guardian -- and far more besides. The Kro’ka begins rooting through Charley’s private thoughts without so much as asking, and when she tries to drive it out, it becomes upset and forces her to relive her death aboard the R101 -- although this death no longer actually happened.

Charley recovers from the traumatic memories to find herself and the Doctor pinned down by a powerful psychotronic force, and the Doctor informs her that the Kro’ka also attempted to turn his memories against him. The Kro’ka then interrogates the Doctor and Charley, asking them what skills they have to contribute in Zone Eutermes should it permit them to pass through the inter-zone barrier. As the psychic coils tighten around their bodies, the Doctor and Charley admit that they’re searching for their home, the TARDIS. Though such things should not exist in this place, the Kro’ka appears quite interested when the Doctor admits that the TARDIS could be used to facilitate space travel. On the other side of the barrier, they will meet others who are equally interested in this TARDIS of theirs, and the Kro’ka thus permits them to pass through, into Zone Eutermes. Experiment number 2.70 has begun...

The Doctor and Charley find themselves in the desert they glimpsed earlier, but upon closer inspection realise that it was once lush with water and life. Though concerned by the Kro’ka’s apparent interest in the TARDIS, the Doctor decides to proceed towards the distant bio-spheres which he assumes are the home of the native Eutermersans. As he and Charley proceed, however, they see a flying car pursuing a fleeing humanoid figure, and as they watch, the car opens fire and apparently guns the fugitive down.

The flying car departs, and the Doctor and Charley investigate to find that its victim is still alive. His skin matches the colour of his surroundings, he has a vestigial exoskeleton, and he appears to be dehydrated. The Doctor and Charley carry the man to a nearby river, where he nearly drowns himself trying to quench his thirst. Obviously traumatised by his experiences, the man, C’rizz, raves about an “elixir” which will make him a king alongside his beloved L’da. He regains enough control over himself to tell the Doctor and Charley that his people have been enslaved by a race called the Kromon; C’rizz himself escaped from Alpha Sphere, the primary Kromon biosphere, but in his haste he left his lover behind, and he now vows to rescue her.

The Doctor advises C’rizz to get some sleep and recover his strength before he acts. He does so, but is woken during the night by a terrible dream. More lucid now that he’s rested, he explains that the Kromon took their slaves to Alpha Sphere to classify them into work groups. L’da and C’rizz were selected to become royals, and were taken to a laboratory to be force-fed an elixir which would cause them to think as the Kromon do. C’rizz’s body rejected the bitter drink, but he saw enough of the Kromon soul to drive him from the biosphere in terror. The Doctor promises to help him rescue L’da, but is personally more interested in the implication that the Kromon are not native to this world. The Kro’ka told him that he would meet people with an interest in spaceflight; perhaps it was implying that the Kromon have his TARDIS.

The next morning, the Doctor, Charley and C’rizz set off for Alpha Sphere. C’rizz and Charley are separated from the Doctor during a sandstorm, and Charley notes that C’rizz’s skin colour is changing to blend in with his surroundings. This comes in useful when a Kromon patrol ship passes overhead, as C’rizz covers Charley’s body with his own and the patrol fails to notice them. The ground then begins to move, indicating that an Oroog, a friendly burrowing creature, is tunnelling beneath them. The Oroog leads Charley and C’rizz to the Doctor and surfaces to say hello, apparently honoured to have helped one such as C’rizz. It claims to be the last Oroog outside of captivity, but even as it speaks, another Kromon patrol arrives and captures the four new friends.

In Alpha Sphere, the Kromon take the Oroog away to be pacified, and interrogate the others to determine how they shall be put to work. The Doctor claims to have a talent for space flight, and the Kromon decide to assign him to the relevant research department. Charley is classified as potential reproductive stock, and C’rizz, under his false name, is assigned to the gardening labour force. The three prisoners are marched to L laboratory, where they will be force-fed elixirs to brainwash them into obeying the Kromon without question. On the way, Charley notes the extreme humidity in the dome, evidence that the Kromon require a great deal of water in order to survive.

In the laboratory, C’rizz volunteers to take the elixir first, and as happened before, his body reacts violently against the elixir and he regurgitates it. While the Kromon are caught off guard, the Doctor and Charley flee. The Kromon scientist, enraged, accuses C’rizz of rejecting the elixir deliberately, and orders the guard commander to take him to the fertiliser plants for execution. On the way, the commander admits that many of C’rizz’s kind cannot digest the Kromon elixirs -- and the same fate has befallen them all.

The Doctor and Charley hide nearby, and follow C’rizz and his guards to a processing chamber for giant fungi. There, they rescue C’rizz from a firing squad by flinging mud at the Kromon as a distraction, and flee back to the laboratory. Charley, aching from the humiliations she’s been forced to endure, overpowers the sole guard and takes his laser gun, but C’rizz rejects the gun when she offers it to him, as if repulsed. Apart from the one guard, the laboratory is deserted; presumably the scientist is off replenishing the stocks of elixir. Fortunately, the computer remains active, and the Doctor enters false data into the system, claiming that he and his friends have been successfully pacified and assigned to their duties. At C’rizz’s request, the Doctor looks up information on L’da, and finds that she has been chosen for a hybridisation experiment and transformed through the application of specialised elixirs. It should not be possible for creatures as different as the insect-like Kromon and the reptilian Eutermersans to cross-breed, but it appears that this is what they’re attempting to do with L’da.

L’da is currently on Level 5 of the sphere, and the Doctor and his companions are on Level 1; nevertheless, the Doctor and Charley set off to help C’rizz find and rescue his lover. They hitch a lift on a freight transport to Level 2, only to find more Kromon waiting at the other end; there are too many to overpower, and Charley thus abandons her gun beside the tracks. When the train pulls to a stop, the Doctor informs the Kromon that he and his friends decided to report to work on their own after their guards were called away to deal with an emergency. Since the computer records indicate that the Doctor and his friends have been pacified, the Kromon guard sends them on their way; the Doctor attends an interview with the director of space research, while Charley is sent to the reproductive reserves on Level 5 and C’rizz (still going under his false name) is assigned to catering duties on Level 2.

C’rizz is shown to the food preparation chambers, where he’s horrified to learn that he is expected to chew and partially digest the Kromon’s food before it is delivered to them. He claims to have a weak stomach, and the supervisor thus takes him to another preparation room to tend to the worms, maggots and flies intended for the higher directorate. This job is no more palatable to C’rizz, and the supervisor, irritated, agrees to assign him to another, temporary position until something more suitable can be found.

The Doctor attends his interview with the director of space research. Despite C’rizz’s claims, it seems that the Kromon are incapable of spaceflight, and that they believe Eutermes to be a different part of their own world. The Doctor outlines some suggestions for a ship capable of spaceflight, but the director does not respond to the Doctor’s description of the TARDIS, implying that the Kromon do not have the craft after all. Furthermore, the director does not understand what the Doctor means by “time travel”, or indeed “time”. Nevertheless, it does find many of the Doctor’s theories interesting, and thus agrees to appoint him as consultant to the Kromon space programme.

C’rizz then arrives, having been assigned to deliver lunch to the Kromon management levels -- although he first stopped off to collect the gun Charley had abandoned. As the space research director chews on his pre-digested food and maggots, the Doctor pops out, claiming that he wants to place an order for dinner. He and C’rizz travel to Level 5, where the controller of the genetics programme is waiting impatiently for his lunch. As C’rizz sets off on his rounds, intending to find and rescue L’da, the Doctor tries to convince the controller that Charley must be reassigned to work as his assistant. The controller initially refuses to give up one of his workforce, but becomes more amenable when the Doctor suggests that he might be willing to consider the controller for a position in a future space colony. He flatters the controller by showing an interest in his work, and the controller proudly shows off the recent generation of Kromon hatchlings; though many are experimental hatchlings, the controller is confident that enough of them will survive to justify future experimentation. Since Charley has not yet begun the transformation process, the controller agrees to reassign her to the Doctor’s department.

The Doctor prevents the grief-stricken C’rizz from killing himself, but the genetics controller then arrives with Charley only to find that the new hybrid queen has been murdered. Enraged, it has the Doctor, Charley and C’rizz all arrested for the crime. After the proper forms have been filled out, the directorate prepares to hear their case, and although C’rizz tries to take full responsibility for L’da’s death, the Kromon have by now discovered that the Doctor has falsified records within their computer system, and he and Charley must answer for that crime as well.

The facts are clear; L’da had been transformed into a valuable queen, and her loss will mean the death of an entire generation of Kromon young unless she is replaced immediately. The genetics controller thus demands that Charley take her place. The director of space research intervenes on the Doctor’s behalf, or rather on behalf of the space research programme, claiming that the risks of allowing the Doctor to continue working for them are outweighed by the benefits should he help the Kromon to acquire the power of spaceflight. The Doctor refuses to discuss spaceflight as long as he and his friends are under threat, and the chairman thus passes sentence. C’rizz is to be strapped to a water wheel on the lower levels, so that his slow death by water torture serves as an example to his fellow slaves, discouraging further rebellion. The Doctor is to be fed elixirs which will turn him into a loyal follower of the Kromon creeds. And Charley is to be taken to the department of applied genetics and transformed into a new hybrid queen.

The Kromon take the Doctor back to L laboratory, where he is strapped down and forced to ingest a series of mind-altering elixirs. The Kromon scientist guides him through the strange visions and sensations that follow, as the Doctor experiences a vision of the humid Kromon homeworld as it was before the Company came prospecting for minerals and stripped the planet bare. The Kromon only survived because of their strength of will and their ability to organise, and though it took them a thousand years, they overcame the Company by adapting its policies as their own. The corporate philosophy became their creed, and when the water at last ran out on their devastated world, they travelled to Eutermes and took over, doing to the people of this zone what had been done to them.

Despite the risks to the Doctor’s mind, the director of space research demands that the process be accelerated. The second draught of elixir stimulates the Doctor’s memories of Mars before it became devoid of life, but the director urges him to his memories of space travel, promising him great rewards -- promotion, personal assistants, even shares in the company. The Doctor bursts out laughing at the paucity of the Kromon’s imagination; having taken over a Company capable of creating a second sun for their planet, the Kromon have let the incredible possibilities of this technology slip through their fingers while occupying themselves with the minutiae of management. Enraged, the director orders the scientist to feed the Doctor another draught of elixir.

Elsewhere, a sadistic Kromon soldier ties C’rizz securely to the water wheel so his suffering will demonstrate to all the futility of rebellion. The guard leaves, laughing, as C’rizz is submerged for the first time. Meanwhile, in the genetics department, Charley is forced to drink the first of the elixirs which will transform her into a hybrid Kromon queen; when she struggles, she is sedated and the elixir is drip-fed to her.

The laboratory technician feeds the Doctor the final elixir, the one which will cause the essence of the Kromon to manifest itself in his consciousness. The laboratory seems to fade away around the Doctor as he comes face-to-face with the Kromon soul, the embodiment of the species’ survival instinct. When he attempts to resist, he is submerged in a river of fire until he begs for the torture to end. The Kromon soul withdraws, satisfied that the Doctor will obey all orders without question from now on. The Doctor awakens and promises to tell the director everything he knows, starting with the construction of an interactive gyroconductorscope, and it soon becomes clear that the director is far out of his depth.

Charley is taken for a walk around the genetics laboratory to ensure that the blood continues to circulate through her body, accelerating her metabolic changes. Disoriented and confused, she doesn’t understand at first why her legs feel so strange, until she realises that they’re becoming more like the legs of a stick insect. Soon, she won’t have legs at all... Meanwhile, C’rizz receives a visitor -- the Oroog, who is now trusted to work without supervision. The Oroog stops the wheel briefly, giving C’rizz a brief respite, and although he’s forced to start it going again when a guard happens by, he assures C’rizz that soon he will return to rescue him for good.

The Doctor outlines his plans for a space-travel machine, but hits a snag when it becomes clear that there is no zeiton-7 ore in this Universe to provide the ship with a means of propulsion. He thus offers to model a substitute fuel using compressed hydrogen. The research director and the chief scientist don’t fully understand the mathematical and chemical formulae which the Doctor is outlining, but they have a target delivery date to meet, and thus reluctantly trust him to model the new fuel on their computers -- albeit under close supervision. The Doctor soon finishes his work, but warns the Kromon that he can’t be sure whether it will work without experimental trial. Nevertheless, the hypothetical model seems sound, and the satisfied director prepares to present the prototype “tardis” craft to the directorate.

As soon as the coast is clear, the Oroog stops the water wheel and chews through the ropes binding C’rizz to it. C’rizz, weakened by his ordeal, allows the Oroog to carry him to safety in a tunnel that the Oroog has hollowed out beneath the fungi garden. There is a guard on duty, but the Oroog lulls it into a false sense of security and then tears it apart. The Oroog apologises to C’rizz for this violence, and is disturbed when C’rizz admits that he also has recently committed acts of violence. The Oroog gets C’rizz inside the tunnel and then fills in the entrance, poking a few air holes and settling in for a nap. C’rizz rests and recovers, and by the next morning he feels strong enough to venture back out in search of the Doctor and Charley. However, Charley’s mutation has continued throughout the night. It is still incomplete, but soon the controller of genetics will be ready to present the new Kromon hybrid queen to the high directorate...

The Doctor flees in the confusion, and literally runs into C’rizz who has disguised himself as a food service worker. Together, they return to the breeding chamber where C’rizz killed L’da, but it remains empty; Charley obviously isn’t ready to take up her duties as a hybrid queen yet. The Doctor stops the sickened C’rizz from destroying the breeding machinery, and after hiding from a guard patrol with guns set to kill, they set off in search of the genetics research chambers. Unfortunately, the chambers are now heavily guarded, and the Doctor and C’rizz have no choice but to retreat to Level 2, shelter with the Oroog and try to come up with an alternative plan.

The enraged Kromon chairman, injured by the explosion in the space research department, sentences the Doctor to death in absentia and reassigns the research director to grade-D clerical duties. The experiment in the genetics chambers is proceeding much more satisfactorily, however, as the Kromon elixirs overcome Charley’s weak attempts to hold onto her humanity. She is soon able to keep down the nutrient drink supplied by the genetics controller, and the scientists thus give her the final elixir, putting her in touch with the essence of the Kromon spirit. Unlike the Doctor, she is unable to resist and is genuinely converted. No longer Charley, she takes the regal name Charlotte, fitting for a Kromon queen, while the scientists prepare to bring the previous queen’s eggs to her to see whether she can support their life force.

The Doctor, C’rizz and the Oroog find a temporary respite in the Oroog’s makeshift underground cavern, but Charley remains in danger, and C’rizz believes that she will have to be put out of her misery, as was L’da. The Oroog explains that the Kromon have diverted all water to their domes, which is why the planet has dried up outside; however, they constantly need more water to survive, and the Oroog and his people have been forced to dig deep in search of it. Sickened by the elixir the Kromon had used on him, the Oroog dug deep beneath the surface of the world -- so deep that he dug into the realm of the Salanders, creatures which live in the fiery heart of the world and give comfort and guidance to Oroog who have lost their way. The Salanders guided him to natural roots and minerals that cleared the confusion from his mind, after which he returned to rescue C’rizz.

The Doctor realises that if the Oroog can collect enough of these roots and minerals, he can free the rest of his clan, and they can shut down the domes’ water supply, distracting the Kromon long enough for the Doctor and C’rizz to try rescuing Charley. If her transformation has progressed too far, however, they run the risk that she is now dependent upon the water supplies, and that cutting them off may kill her. Nevertheless, the Oroog sets off on his mission while the Doctor and C’rizz settle in for the difficult part -- waiting. C’rizz can’t stop thinking about L’da’s death, and the Doctor can’t stop worrying about the Kromon; if they acquire spaceflight, their wide array of elixirs could make them a threat to any planet within their reach. Eventually, he and C’rizz decide that they’ve waited long enough, and as they set off for the upper levels, they see the water fountains begin to run dry; presumably the Oroog has succeeded.

In the genetics laboratory, Charley concentrates upon L’da’s eggs as they hatch into larvae. The fact that they are a previous queen’s eggs gives her some difficulty, but Charley finds that she can sense the larvae’s confusion, and gives them the direction they seek. The genetics controller watches in delight as the larvae form into lines and begin carrying food to their new queen. The experiment is a success, and once the controller has reported to the directorate, Charley will be installed in the breeding chambers. However, the controller’s report to the board is interrupted by the arrival of the panic-stricken guard commander, who reports that the flow of water through the biosphere has stopped.

The Doctor and C’rizz run into a patrol of Kromon repairing the emergency water system controls, but their Oroog friend arrives and tears the patrol apart, once again apologising to C’rizz for the violence. He accompanies the Doctor and C’rizz to the upper levels, where they see that the royal Kromon are dying off, deprived of the constant water supply they need to survive. Together, they proceed to the breeding chambers, where the attempt to install Charley in the breeding pool has hit a snag due to the lack of water pressure in the fertilisation jets. C’rizz guns down the Kromon scientists, including the genetics controller, but the Doctor stops him from doing the same to Charley. There’s still a chance to save her, if they can get her to a pool of water and supply her with the drugs provided by the Salanders. The directorate guard bursts in to stop them, but stutter to a halt and collapse before they can open fire. The entire Kromon hive is linked to the minds of the royals, and now that they’re dead, the others have no direction and cannot even move.

The Oroog helps the Doctor and C’rizz carry Charley to the reservoir, and gives her the roots and minerals which helped clear his mind of the Kromon influence. C’rizz opens the doors of the dome and releases his fellow Eutermersans from slavery while the Doctor watches over Charley; her body is returning to its human form, but what will happen to her mind? They may not have time to wait for her to recover, as the Doctor fears that the other biospheres will send forces to retake Alpha Sphere -- but fortunately, Charley does wake, with little memory of her traumatic transformation. As the Doctor and his friends leave Alpha Sphere, the Oroog lays the Doctor’s fears to rest; once it was clear that Alpha Sphere had fallen, the Oroog’s people cut the water supply to the other spheres as well. Now, water is returning to the surface of the planet, and the Oroog, bidding a fond farewell to his new friends -- and particularly C’rizz -- sets off to help the Eutermersans reclaim their world.

The Doctor and Charley now have more mysteries to solve. What is the nature of the company that devastated the Kromon homeworld? Are they the ones who created these mysterious interconnected zones? And what has happened to the TARDIS? They will have help on their quest; with L’da dead, there is nothing to keep C’rizz here any longer, and he’s come to realise that there is more to life than he had previously imagined. He thus asks if he can accompany the Doctor and Charley on their travels, and they accept the offer of his company. The interzone barrier then opens up for them, and the Doctor, Charley, and C’rizz step through.