The planet Cimmeria IV is rich in minerals and has no natives, as far as anybody knows. Thus, a survey team has been sent to the eternally dark planet to install EPUs, Energy Projection Units, to prepare for full-scale exploitation. The EPUs are now in position, and although team members Ferras and Haliard gently mock their leader Orllena’s earnestness, it’s clear that the three teammates respect each other’s abilities. However, when they attempt to power up the EPUs, the base shakes on its foundations, even though those foundations are buried hundreds of feet beneath the planet’s surface. The shaking eventually stops, but the base’s power systems are failing -- and as this planet has no sun, that means the lights are going out for good. Ferras tries to fetch the emergency lamps, but something brushes past him in the darkness, and he realises there’s something else in the base with them. Panic-stricken, Orllensa sends a Mayday signal with the last of the emergency power, but then there is a rising hum and the three teammates begin to scream in agony as an alien voice whispers to them. “Embrace the darkness. You are safe forever...”

The Doctor is making tea when he detects a nearby fleet of Type 70 TARDISes. Choosing not to entangle himself in whatever bureaucratic confusion the Time Lords have in store, he tries to slip past quietly, only to run into serious temporal turbulence when he tries to put his ship in hover mode. The TARDIS emerges in real space in the Cimmerian system, and to the Doctor’s surprise, Charley proves that she’s been paying attention to him when she operates the console data bank in order to learn more. According to legend the system’s star disappeared one day, which is where the system’s name comes from -- “Cimmerian” meaning “of darkness”. The Doctor sets the TARDIS to drift forward in Time at a half century per second, and he and Charley see a number of mysterious objects approaching the sun, which then goes dark.

Intrigued, the Doctor tries to go back and watch the sun’s disappearance in real time, but once again he has trouble getting the TARDIS to move through Time, and it materialises fully in the system. As the Doctor attempts to reset the co-ordinates, the TARDIS is located by Rescue Operational Security Module G723, which has arrived in response to Orllensa’s distress signal. Unable to identify the TARDIS, the ROSM unit teleports it aboard for further examination while it continues attempting to contact the base on Cimmeria IV. Unable to do so, it prepares to land and investigate the loss of contact personally. Meanwhile, the Doctor and Charley emerge from the TARDIS, only to find themselves sealed behind a force field. They can only hope that whoever transported them here isn’t hostile...

In the base on Cimmeria IV, Haliard is crying and he won’t stop. Ferras can’t find him in the dark, and he’s surprised when Orllensa finally responds to his calls; he’d thought that she had been taken away, but she was just numb with shock. Haliard still won’t stop crying, and Ferras is starting to lose his composure as well, as the sobbing reminds him of the desperate agony he has just experienced. Orllensa knows she’ll never forget the pain for as long as she lives. But how long will that be?

ROSM completes its preliminary scans of the Doctor and Charley, and sends one of its roving assault units to interrogate them. As they attempt to explain their presence, however, it completes its analysis of the preliminary scans, which indicate that Charley is a potential biohazard and must be eliminated. Charley tries to retreat to the TARDIS only to find that ROSM has placed a force field around it. The Doctor tries to convince ROSM that it is mistaken, but fails; however, use of the sonic screwdriver disorients the assault unit just long enough for Charley to flee. ROSM recovers from the attack and confiscates the sonic screwdriver, but recognises that it was obviously designed by a highly sophisticated intelligence and therefore concedes that the Doctor may be worth listening to. However, it remains suspicious, as the nature of the emergency situation on the planet is still undetermined, and as far as it knows the Doctor and Charley are the only other life forms in the system apart from the base crew.

Charley tries to dodge the other roving assault units, but there are too many and she is soon recaptured. Before ROSM can terminate her, however, the Doctor manages to explain that the “potentially dangerous cellular clusters” it has detected are naturally occurring. Charley comes from a culture that doesn’t practice genetic modification of its embryos; thus, the impurities which ROSM is detecting do not pose a danger to the base crew, and don’t even pose an immediate threat to Charley herself. ROSM accepts this, but then reveals that secondary scans have revealed other unusual data about Charley which its instruments are unable to process properly; therefore, it intends to proceed with her termination to be on the safe side. Charley manages to wriggle out of the assault unit’s grip while it’s considering the Doctor’s protests, however, and flees through the nearest hatchway, closing it behind her. However, she’s just hidden inside an escape pod -- as she learns when it launches, saving her from the immediate danger but separating her from the ship, the Doctor, and the TARDIS.

The Doctor is concerned for Charley’s safety, but ROSM explains that the life pod will home in on the nearest planet -- specifically, to the airlock of the base on Cimmeria IV. This gives the Doctor some time to convince ROSM that Charley doesn’t pose a threat... but if something has happened to the people on the base, then it may happen to Charley as well. ROSM allows the Doctor onto its control deck in case he has anything further to add, but keeps him confined behind a containment field to prevent him from interfering as it carries out its programmed instructions. As it prepares for landing, it scans the Cimmeria IV base and detects a large quantity of unidentifiable particles which are inhibiting its scanners’ efficiency. It thus generates a particle dispersal beam to clear the air before sending in its remote units. The life pod is about to dock with the base...

As the Doctor waits impatiently for word of Charley, ROSM detects an increase in the density of particles within the base; however, it can’t increase the power to its dispersal beam until it actually lands and links up to the base’s airlock. Inside, Charley is at a loss to deal with Orllensa and Ferras, who aren’t responding very well to her news. When the light from the pod starts to dim, and Charley begins to feel pain in her eyes, all Orllensa can do is listen -- and pity her for what is about to happen.

The Doctor hopes that ROSM will classify the unknown particles in the base as a greater threat than Charley, but ROSM is still having trouble analysing the results of its scan and reseves judgement. ROSM links up to the base airlock, increases power to its dispersal beam until the particle density is at minimum, and allows the Doctor to accompany one of its assault units into the base. It also returns his sonic screwdriver, confident that it can now defend itself should the Doctor try to use it as a weapon again. The computers in the base are irridated with the same unknown particles, but ROSM eradicates them and turns some of the lights back on. The Doctor sets off to look for Charley, and ROSM allows him to go -- although if he continues to act on his own, shooting him may yet become an option. The Doctor, searching for Charley, instead runs into Haliard, who has been driven mad with fear and pain and who runs screaming into the darkness when the Doctor tries to approach him. Elsewhere, whispering alien voices plan their response to the return of the light. If the newcomers do not embrace the darkness, they will die.

The Doctor meets Orllensa, and is horrified when he gets a good look at her face. She seems dully resigned to her fate, but when the Doctor presses her for details she snaps angrily at him, suggesting that she may in fact be in shock. Nevertheless, she leads the Doctor back to the airlock where Charley docked with the base -- although she’s lost track of Charley, who fled when her eyes began to hurt. Elsewhere, Charley is staggering through the base in pain; her eyesight is blurred, but not completely gone. She too hears Haliard cackling madly to himself, but he flees in terror when they hear a chittering in the darkness. A blurred shape approaches Charley, who realises that it is a Cimmerian. Frightened and angry, she demands to know why the Cimmerians are trying to burn out their eyes, but it doesn’t seem to understand the question -- or know what “eyes” are. It tells Charley that light is bad and that she must embrace the darkness, and begins to release a particle field in her direction -- but before it can complete its work, ROSM’s assault unit arrives, challenges it, and opens fire.

The Doctor hears the gunfire and investigates, to find that ROSM has only stunned the Cimmerian and has chosen to keep Charley alive for further analysis. When it arrived, the alien creature was releasing an energy field in Charley’s direction, apparently generating particles similar to those which damaged the base’s lighting; thus, Charley has become a vital part of ROSM’s investigation. The Doctor leads Charley and the assault unit back to the secondary airlock to meet Orllensa and Ferras, but they take the stunned Cimmerian with them. The other Cimmerians must respond to this, for if the newcomers discover the Cimmerians’ secrets they will bring suffering to this world once more. They cannot be permitted to leave.

ROSM activates further assault units and scans Charley in more detail. As it does so, Charley realises that her eyesight seems to be returning. Orllensa has become quite bitter and angry about what’s been done to her, and she’s lashing out at everyone, especially the unconscious Cimmerian. Charley tries to describe the creature as best she can, but her eyesight is still poor; however, the Doctor claims that it looks small and childlike, surprisingly vulnerable, and clearly adapted to a life in darkness. Orllensa wants nothing more to do with the planet, but there’s still Haliard to think about. He’s hiding in the base, trying to convince himself that he doesn’t exist and therefore can’t be hurt, but then the Cimmerians find him. They seem puzzled when he rants at them for taking his eyes; what are eyes? Why can he not taste the darkness? They approach him, questioning if he is in pain, and tasting his fear as he screams in terror. Soon they know what to do about it.

Ferras hears Haliard screaming, and ROSM decides to classify this as a hostile attack situation and respond in kind. The Doctor advises care in contacting this unknown species, but ROSM’s responses are dictated by company policy -- and since the lives of base personnel are protected by warranty, it must respond to the Cimmerians’ actions with force. The Doctor, frustrated, tells Charley to take the others back to the rescue ship while he tries to find out what the Cimmerians really want -- but ROSM refuses to let him interfere, and threatens to kill him if his behaviour contradicts its programmed imperatives. The Doctor strikes a compromise by convincing ROSM to let him take the Cimmerian captive onto the ship for further study -- much to Orllensa’s distress, as she doesn’t want to be anywhere near it.

Once aboard the ship, the Doctor tries again to speak to ROSM, but it’s fully occupied by its attack on the Cimmerians and refuses to respond. The Doctor thus goes to the bridge to speak to it directly, leaving Charley to care for the Cimmerian. Orllensa is infuriated that the Doctor and Charley are being so considerate of the creature that burnt out her eyes, and makes no effort to hide her anger. However, Charley realises that her eyesight really is improving -- and that this started to happen after the Cimmerian spoke to her. Curious, she asks Ferras about his reason for being here, and he starts to explain about the EPUs, artificial suns which the company planned to place in orbit around the planet. Orllensa orders him not to tell the strangers anything, and Charley snaps, telling Orllensa that, trauma or not, she’s still a very unpleasant person. She storms off to find the Doctor and tell him about her eyesight, but this means leaving Orllensa and Ferras alone with the Cimmerian... and Orllensa can’t bear the sound of the creature breathing in her presence.

ROSM’s assault units make contact with the Cimmerians and demand that Haliard be returned. The Cimmerians refuse, and ROSM thus opens fire on them. The Doctor, watching from the bridge, is infuriated; does ROSM really intend to blast its way through a whole alien civilisation just to rescue one man? But the question becomes moot when the assault units detect an increase in particle density -- and begin to malfunction. The Doctor urges ROSM to disconnect its control link to the assault units, but he’s too late; the particles are being transmitted along the transmission wave and are already affecting ROSM’s central processor.

The Doctor manages to convince ROSM that it is in danger and that he has been giving it good advice, and it therefore listens when he advises it to detach itself from the base and take off. Unfortunately, the advice comes too late, for the Cimmerian particles have shut down the ship’s engines. However, the system failures containment fields also shut down, freeing the Doctor and Charley. The Doctor decides to return the Cimmerian captive to its people in the hope that they will then stop their attack. Soon after he and Charley leave, Haliard contacts ROSM, claiming to have an important message from the Cimmerians, but ROSM no longer trusts its perceptions and therefore refuses to risk taking any action. Since opening the airlock to let Haliard in would also allow more particles to enter the ship, it refuses to discuss the matter with him, and terminates communications.

The Doctor and Charley return to the waiting room, only to find that the Cimmerian has gone. Orllensa claims to have done nothing to it -- much as she wanted to -- and yet neither she nor Ferras heard the door open before the Doctor and Charley returned. The Doctor reveals that Charley’s eyesight has been fully restored, and it seems that the Cimmerian is responsible, for it happened after she was exposed to an energy field emanating from the alien’s body. Orllensa scoffs at this suggestion; why would the Cimmerians burn out their eyes and then restore them? But the Doctor points out that the Cimmerians don’t seem to have eyes themselves, and that it spoke to Charley of “tasting”, as if this were its primary sense. What if the humans’ eyes were not burned out deliberately, but accidentally -- and what if the Cimmerians can fix the damage? Orllensa is characteristically skeptical... but eventually even she must concede that the situation is desperate, and that under the circumstances, it may be worth making a blind leap of faith.

First, they must find the Cimmerian, and as it seemed to be drawn to Charley’s suffering, the Doctor suggests splitting up. He will go with Ferras while Charley accompanies Orllensa, and they’ll hope that the Cimmerian is drawn to those without eyes. As they search the ship, the Doctor asks Ferras what he’s doing on the planet, and although Ferras suspects the Doctor is trying to get his mind off his troubles, he explains about the ring of artificial suns. They had finished installing them, and where just about to activate them when the Cimmerians struck, shifting the central EPU out of alignment. The Doctor and Ferras then find themselves back at the control deck, where they hear Haliard trying to convince ROSM to surrender. Presumably the Cimmerians have brainwashed Haliard somehow, but as long as ROSM has shut down communications, they can’t speak to him or open the airlock for him. As ROSM’s condition continues to worsen, the Doctor decides to activate the EPUs by remote control; if they’re lucky, the sudden blast of heat and light will overwhelm and destroy the Cimmerian particles.

Charley tries to draw Orllensa out of herself, and Orllensa admits that Ferras and Haliard were the best teammates she’d ever had. She’d been suffering a run of bad luck before meeting them, as her previous teams had run afoul of planetary natives and interstellar terrorists. Charley admits that she just followed the Doctor here because he was curious, but when Orllensa asks her if she’d prefer to have stayed at home, Charley is surprised to realise that she considers the TARDIS her home now. Charley then hears a chittering in the darkness, and sets off to investigate, telling Orllensa to wait. The terrified Orllensa hears the creature approaching, and begs for mercy as it releases another energy wave towards her... but there’s no cause for alarm. When Charley returns she finds Orllensa singing a gentle lullaby to the creature which has just restored her eyes... The effort appears to have exhausted it, and it informs them that what it has done is forbidden. Now the Solarians will return; creatures from ancient times, who will see the light and destroy them all...

The Doctor locates a manual override, accesses the data files on the EPUs, and learns all he needs to know about the procedure to illuminate the planet’s surface. The twelve EPUs in orbit are slaved to the central unit, and will automatically adjust to remain in alignment -- which means they still have a chance. Haliard is growing ever more desperate to warn the others of the danger they face -- the life support systems on the ship are soon going to shut down. The Doctor, who can’t open the airlock in any case, focusses on the job in hand, and manages to send an activation signal before ROSM’s communication systems break down. Unfortunately, it’s at that point that Charley and Orllensa return with the exhausted Cimmerian and their story. Perhaps when it recovers it can heal Ferras, but that’s beside the point; the point is that the Doctor may have just made a terrible mistake. There are many races in the Universe which go by the name of Solarians, and he’s unsure who the Cimmerians’ ancient enemies might be -- but it sounds as though the Cimmerians deliberately darkened their sun in order to save themselves from being wiped out. And now the Doctor may have illuminated their planet once again.

ROSM tries to interface with the tracking satellites and get a clear loook at the new arrivals; in the meantime, it reactivates its communications systems, and the others finally get a good look at Haliard -- and he at them, since his eyes and sanity have been restored. The Doctor theorises that the Cimmerians are a race of healers, and the hypothesis seems borne out when the Cimmerian on the control deck touches Ferras and causes his eyes to grow back. Sadly, the effort costs the weakened creature its life, and the Doctor realises that it was compelled to heal whatever the cost to itself. Upset, he advises Orllensa to prepare for takeoff and to destroy the artificial satellites, restoring darkness to the Cimmerians. In the meantime, he will take the body of the dead Cimmerian back to its people. He tells Charley to stay where it’s safe, and she only agrees when he promises to return shortly.

Unfortunately, ROSM still hasn’t fully recovered, and its memories of recent events are hazy; all it knows is that the Doctor caused damage to its systems and accessed restricted files, and it doesn’t accept the Doctor’s claim that he was trying to save everybody’s lives. Fed up, the Doctor opens the airlock with his sonic screwdriver, letting himself out and Haliard in. On his way out, the Doctor asks Haliard about the Cimmerians, and is upset to learn that they will be too weak to unleash another wave of particles for quite some time. Blaming himself, the Doctor asks Haliard to take care of Charley if he doesn’t return.

On the bridge, ROSM finally gets a clear picture of the Solarian ships, and Charley recognises them as the same ships she and the Doctor saw approaching the sun when it went out centuries ago. Haliard arrives, but when he tells Charley what the Doctor said, she’s furious and storms out to join the Doctor outside. The Doctor claims that he just wanted her safe, but she knows better; he feels guilty and wants the Cimmerians to punish him for his potentially genocidal mistake. The Cimmerians then arrive, and use their molecular control technology to transport the Doctor, Charley and the dead Cimmerian down through several feet of solid rock; this is how the Cimmerian escaped from the lounge without opening the door. As the Doctor and Charley try to learn more about the Cimmerians and the Solarian threat, there’s a moment of confusion which clears when they remember that the Cimmerians don’t call themselves “Cimmerians” -- that’s a name imposed upon their system by other races after the sun went out. He tries to apologise for his actions, but the Cimmerians admit that they too harmed the humans in error. It seems that the tragedy to come was born from misunderstanding. Hoping to correct his mistake, the Doctor asks to learn more about the Cimmerians, and they agree to let him taste their history.

Back on the rescue ship, things are becoming desperate. ROSM’s repair and safety diagnostics are underway, and they appear to have a 45-minute margin of safety... but the Solarian ships are accelerating. As they draw nearer, Haliard realise that they’re powered by solar sails -- which means that the closer they come to the artificial suns, the more light they pick up and the faster they can go. ROSM tries to cut as many corners as possible, but it’s compromising vital safety checks in the process. Orllensa, frustrated, orders him to skip as many safety routines as soon as possible, and sends Haliard out to warn the Doctor and Charley of the danger. As soon as everyone’s on board, they’re taking off.

The Cimmerians place a simmering stew before the Doctor and Charley, who realise that “taste” is the Cimmerians’ most important sense. This “stew” is a medium in which their history is stored. Charley again turns aside the Doctor’s attempt to protect her and tastes the history along with him. Whether history or legend, when the Doctor and Charley eat the stew they perceive the deaths of millions of Cimmerians, caused by the Solarians’ arrival. A terrible plague had visited the Solarians, and they came to this world, their solar sails catching the light of the sun -- but though the Cimmerians healed their sick, the demands of the Solarians became too great and the Cimmerians began to die. But the Solarians kept coming, and to save themselves, the Cimmerians had no choice but to shut out the sunlight and turn the Solarians away. The effort exhausted those who remained, and they slept for years, evolving and adapting to the darkness, and becoming as they are today. Now it’s all coming to an end, and the Doctor holds himself responsible -- but the Cimmerians regard this as justice for their choice to put their own needs above those of the Solarians. Haliard arrives, and the Cimmerians allow the Doctor and Charley to depart; however, the Doctor vows to do what he can to save them.

The Solarian ships are too close for comfort, and as soon as the Doctor, Charley and Haliard are aboard, Orllensa overrides all safety protocols and orders ROSM to blast off. It has no choice but to obey. As they enter orbit, the Doctor advises Orllensa to try avoiding another misunderstanding by contacting the Solarians peacefully, but then the Solarians open fire on the rescue ship. Orllensa thus orders ROSM to destroy the artificial suns -- but it refuses, and shuts down its weapons systems to prevent her from damaging company property. Frustrated, the Doctor realises that they have no choice but to fire on the Solarian ships... but ROSM has decided that since Orllensa tried to destroy company-owned satellites, she cannot be trusted with weapons control, and refuses to let her open fire even as the defense shields begin to disintegrate. They probably should have let ROSM complete those diagnostics...

Orllensa transmits an unconditional surrender, and as the Solarians prepare to dock with the rescue ship’s airlock, the Doctor leads the others to his TARDIS. It’s now their only means of escape -- or at least it would be if ROSM hadn’t placed it in a containment field. Its origins are still unknown, and the Doctor and Charley are still under investigation; thus, ROSM refuses to let them, or anyone else, into the TARDIS. It seems that ROSM, going by the letter of its instructions, is going to get the people whom it came to rescue killed instead. The Solarians arrive in miniature, self-mobile tanks, and angrily order the humans to explain their trespass on the Solarians’ world. Charley for one is infuriated by this arrogant claim to the Cimmerian homeworld... and some moments of confusion follow as the Solarian slowly realises what it’s being told. It is horrified to learn that there are still people living on the planet, in the darkness -- people who nearly destroyed themselves to escape from the suffering the Solarians caused. Stunned, the Solarian opens up his tank and emerges, revealing a shockingly familiar form -- albeit one with eyes. The Solarians aren’t warriors after all, but archaeologists in search of evidence of a lost civilisation -- a people once abused by their ancestors. For the Solarians and the Cimmerians are of the same species.