According to legend, a wise old king once passed on into the world of infinite darkness, and before he died, he advised his son and heir to rule his subjects with love as well as fear. But his son chose to ignore his father’s advice, and demanded that his subjects subsume their wills to his own. Even the animals were ordered to obey their king’s commands. Soon all in the kingdom lived in perfect harmony, according to the whims of their lord...

The TARDIS is about to enter a new universe, and Charley is disturbed by the noises coming from the console -- and by the fact that the Doctor is hiding underneath it. He claims that there is nothing outside the TARDIS now, and that’s what he’s hiding from: nothing, everything, death, pain, fear, and perhaps even Charley. He does not believe that Charley is really here, insisting that he left her safely behind and that she would never betray him by coming along with him. He then begins to convulse; Time does not work the same way in this universe as in the universe of the Time Lords, and the Doctor’s temporal senses are shutting down, leaving him blind to the passage of Time. Suddenly he’s experiencing it just as humans do; there is no way to travel back and forth through it, and all he can do is experience it as it passes on, never to return.

The background hum dies away, as if the TARDIS has realised there is no place for her in this universe. The console room begins to dissolve into darkness, and to Charley’s horror, the Doctor seems content to let it happen. Charley refuses to surrender, and insists that they leave the TARDIS before it is destroyed. The TARDIS doors open, giving them a chance to escape, but the Doctor freezes in fear, unwilling to risk going out into the blinding light of a whole new universe. As he hesitates, the darkness surrounds him, and for a moment he’s lost, terrified and alone in the dark. But Charley enters the darkness, takes his hand, and leads him out of the TARDIS as the rest of the console room vanishes into the pitch black.

Outside, the light itself is blinding, so bright that Charley can still see it with her eyes shut. She begins to panic and tries to flee back into the familiar safety of the TARDIS, but it dematerialises, leaving her and the Doctor stuck. And as the Doctor well knows, there are no second chances in this new universe; they’ve made their choice and must stick with it. The Doctor tells Charley to take his hand so she doesn’t get lost, and they start walking, hoping that they will get somewhere.

As they walk, Charley finds that her eyes are not adjusting to the light. The Doctor points out that they don’t know the physical laws of this universe; perhaps their eyes will never adjust at all. Perhaps their brains aren’t wired to process the visual stimuli provided in this universe. In fact, it’s surprising they can even breathe here. When the Doctor asks Charley to describe the smell of their surroundings, she finds them faintly musty, but when the Doctor prompts her she recognises the smell of fruitcake -- or so she thinks. In fact, their surroundings smell of nothing at all, and her brain is just filling in the gaps. They cannot see, hear, smell, taste or feel anything at all in this new world, apart from each other.

Charley and the Doctor seem to hear the TARDIS materialising nearby, but the Doctor fears that it’s only a shared delusion. Nevertheless, Charley rushes off to investigate and trips over something on the floor. The blinding light fades momentarily, revealing that she’s fallen over the body of a creature like a giant amoeba. It appears to be dead, but the Doctor and Charley can hear a squeaking voice, apparently asking them for help. The voice repeats its request at various pitches and volumes, and the Doctor and Charley then hear the sound of the TARDIS once more. An invisible force tears the amoeba creature apart, and then the brightness returns and there is once again nothing to see.

Though the people and the animals obeyed the King’s whims without question, he still was not content. At his command, the waves, the wind, and even Time itself were brought to heel. But music refused to be disciplined or constrained, and in his rage, the King ordered it banished from his kingdom forever, into the outside world of infinite darkness...

The Doctor and Charley have been walking in silence for some time, and Charley finally begins to hum Frere Jacques, unable to bear the silence. The Doctor asks her how long they’ve been silent, and is amused when she suggests it’s been half an hour; actually, they’ve been walking in silence for over 32 hours, and in total have been walking for the better part of a week. Stunned, Charley realises that she can barely feel the Doctor’s hand in hers, even when he squeezes it as hard as he can. With no external stimulation, their senses are atrophying; the Doctor can only tell the time because he’s been counting his heartbeats.

Charley is somewhat relieved to learn that the Doctor had a reason for being silent, at least until he reveals that he is angry with her after all. He believes that she made the wrong decision back in the TARDIS, and that they should have entered the darkness instead of the light. He’s also puzzled by their continued survival in this entirely alien environment, and suspects that somehow it’s been arranged for them. But if someone is deliberately keeping them alive, why is it doing so, and why has it removed all that makes that life worthwhile? Charley tells the Doctor that even this is bearable so long as he’s with her -- but the Doctor doesn’t wish to discuss that, and they resume walking in silence.

Only a few seconds later, the silence is broken by humming -- but this time, though it sounds like Charley, it isn’t. The Doctor tries speaking to the new presence, and it replies to his soothing tone by repeating the words which the Doctor and Charley have spoken to each other since they arrived. However, the words themselves convey no meaning; the creature is responding to the intonation and the emotion expressed through them. It seems to be asking a question of the Doctor and Charley, but it’s unclear what it wants, as it only repeats the words “I love you” over and over until they lose all meaning.

The Doctor now has a puzzle to solve, but Charley soon has something else she wants to discuss, as the Doctor refers to the “embarrassing” moment when she claimed to love him. She meant it, and it seems the Doctor doesn’t share her feelings. Irritated, the Doctor stops trying to work out the nature of the environment in which they’re doomed to spend the rest of their lives so Charley can discuss her feelings. He claims to consider her love inappropriate and pointless, and though he once told Charley that he loved her, those feelings are now as dead as all of his other senses, and he only said them to comfort her when he thought she was going to die. Charley, furious, orders him never to speak those words again unless he really means them.

The Doctor promises not to toy with Charley’s emotions, but realises that this is what someone else is doing. Their prolonged sensory deprivation has left them in a suggestible state, open to emotional manipulation. Music is known to affect the emotions, and there’s music in every sound, even the rhythm of footsteps or heartbeats. Just by being here, the Doctor and Charley are making music together, and that music is being turned against them. However, the Doctor senses no malice in the creature’s attempts to communicate with them -- at least, not yet. Perhaps it’s merely toying with them, like a child with a new plaything. But if they resist its attempts to manipulate them, they run the risk of boring it -- and what will it do with them then?

As the Doctor and Charley ponder what to do, the voice hums Frere Jacques and giggles, and the blinding light begins to decrease. Suddenly, the Doctor and Charley can make out some of the shapes around them -- and Charley collapses, exhausted, as her senses return and her body reacts to the fact that she’s been walking non-stop for days without food. As she rests, the Doctor explores their surroundings and determines that they’re in a moderately curved tube of glass; however, it’s still too bright for him to see what lies outside.

Another dead amoeba creature lies nearby, and the Doctor borrows Charley’s brooch to dissect it. This creature appears to be more highly evolved than the first, but even as the Doctor examines the body, something invisible cuts across its throat, accompanied by the sound of the TARDIS engines. The invisible creature then turns on the Doctor, but as it gnaws at his neck, Charley desperately begins to sing Frere Jacques once more, and the creature releases the Doctor. The Doctor realises the truth; this creature is not their captor, and it’s not just using sound against them as a weapon. It is sound itself, living music assembled from every sound the Doctor and Charley have made since arriving in this environment. They can’t stop themselves from making sound, and the music cannot be reasoned with, as it’s a simple organism driven by simple animal desires such as hunger.

When the foolish King banished music from his land, life became a drab thing with no harmony or rhythm. The slaves could not sing their troubles away, the torturers were haunted by their victims’ silence, and even the King’s wives had no love in their voices. Finally, the King pardoned music, but it did not return, nor did the messengers he dispatched into the outside world of infinite darkness. The King was thus forced to make the journey himself by slowly draining the blood from his body, and as he lay halfway between life and death, he begged the music to return to his land...

The Doctor and Charley have fallen into a routine. They walk together, hand in hand, for an indeterminate period of time until the music arrives to “perform” for them; they demonstrate their approval, like parents to a talented child; then, the brightness dims just long enough for them to slice open one of the dead alien animals and eat. Occasionally, the music gnaws at their throats, but it does not cause serious harm; furthermore, the corpses have been evolving as well, and not only do they appear more mammalian and taste more appetizing, but their blood now acts as a salve on the Doctor’s and Charley’s skin.

Theirs is a tolerable existence, but when Charley claims to appreciate it for what it is, the Doctor harshly reminds her of all she’s lost. They may be treated well here, but they’re still slaves, and their pain and regret is the part of them that made a full life of mere meaningless existence. Charley admits that her brooch, which is now clotted with blood after cutting open countless alien bodies, was a family heirloom, passed on from her grandmother to her mother and on to Charley herself; however, Charley will never be able to pass the brooch on to her own daughter, for she will never have one.

The brightness fades once more, but Charley is in for a shock; she and the Doctor have been holding hands for so long that they’ve fused together. Charley begins to panic, but the Doctor calms her down, warning her that her fear is affecting him as well. He tries to counter the fear with reason, and deduces that this environment was designed to accelerate the process of evolution -- in which case, the alien corpses are all that remains of the species which should have evolved here had the Doctor and Charley not brought the music with them. If this is true, it means that their true captor is something with a reasoning intelligence, and also implies that there must be a way out of this laboratory environment. The Doctor starts walking again, but Charley digs in her heels, forcing him to stop. Just as he saved her from her fear, she saves him from his burst of unfounded enthusiasm, by reminding him that they’ve already been walking for weeks, perhaps months. This tube may be as large as the universe itself, and even if the Doctor’s deductions are correct, they’re still no closer to finding a way out.

The Doctor concedes Charley’s point, and when she claims that she can feel him calming down, this gives him an idea: if evolution is pushing them together, perhaps they can speed up the process. The Doctor and Charley push their hands together, and their hands and arms flow into one, enabling them to share each other’s senses and see more clearly. The Doctor eagerly peers through the side of the glass tube they’ve been walking through... and, to his horror, sees that the tube is in the shape of a torus. He and Charley have been walking in circles all this time, and there is no way out.

Stunned, the Doctor lashes out at Charley, blaming her for their situation; were it not for her, he’d have a whole universe to explore, but instead he’s trapped in this endless half-life. He reveals to the stricken Charley that the Time Lords believe his companions to be memento mori, reminders of the Doctor’s own mortality -- and if this is so, then Charley, a companion who was already dead, would be the ultimate fashion accessory. And yet, when forced to choose between Charley’s life and the entire web of Time, the Doctor sacrificed his own life to save the Universe so she could live. But then she followed him into this universe, giving up her life so she could be with him, just as he gave up her life for her sake. Their love for each other has condemned them both to death, so what was the point of it? The Doctor gave up everything to save Charley, but she cheated him of that success -- and now that he’s stuck with her, literally as well as figuratively, he wishes he’d never met her at all.

Now that this is out in the open, the Doctor and Charley have little more to say to each other, and the Doctor thus prepares to cut the flesh off the dead animal for their next meal. But when he examines the animal closely, he reveals that it’s evolved again -- and since it’s taking its cues from its environment, it has evolved into a copy of Charley. Repulsed, Charley realises that this means they’ve effectively been eating the only daughter she’ll ever have, and she refuses to eat any more of her “offspring,” though this may mean her death from starvation.

When the foolish King awoke, he found that music had returned to his kingdom -- a savage, bitter and violent music that tore the throats out of his subjects as they sang. As the birds fell from the sky and the waves crashed and died on the shore, the music told the horrified King that, in the outside world of infinite darkness, it had learned to inspire fear as well as love. And so the music killed the King and became the new lord of the entire world...

The burst of sound and music suddenly stops. The Doctor’s vocal cords are exposed but intact, which is all the music ever wanted; they thought it was chewing at their throats, but only in the sense that a breastfeeding baby may bite too hard. The Doctor has now given it all of the sounds it ever desired, hoping to accelerate its development so he can reason with it. But when the alien animal disintegrates before his and Charley’s eyes, he realises that he’s made a terrible mistake. This environment is designed to accelerate the process of evolution, and any creature which is out-evolved by its competitors must die out. Charley’s hand begins turning to stone as the Doctor speaks; unless he acts quickly, he and Charley will be out-evolved as well. The Doctor thus urges Charley to continue the process which has already begun by pushing herself into his body; they must each give up their individual self for the sake of the other. The Doctor and Charley push together and merge into a single body, but the change is too much for Charley to handle. Swamped by the new sensations, she loses her sense of self, and her screams fade away, leaving the sound of a giggling child...

Charley regains her senses to find herself in the drawing room of her parents’ house. Much to Charley’s surprise, her daughter arrives, speaking with the voice of the Doctor and thanking her mother for giving it the gift of life. Now Charley must hand over her brooch and make way for her child, as did her parents and grandparents before her; this is the nature of evolution, as the child understands it. Charley prepares herself to die so her perfect daughter may live, but when she remarks how beautiful the brooch will look when the child wears it, she realises that her daughter does not understand what the brooch is for. To the child, it’s just a symbol of tradition to be passed on from generation to generation, but Charley insists that it’s more than that -- and if the child doesn’t understand that, then it’s not old enough to deserve the brooch. Charley takes the brooch back, and the furious child abandons her, setting off to find its father and make him die for its sake.

The Doctor seems to be back home in the TARDIS, with the entire Universe to explore once again -- but then Charley arrives to remind him that he’d promised to give up his life to save that universe. Shouldn’t he be getting on with it? The Doctor realises that this is not Charley, but the music creature, his child, transformed into an amalgam of all of his past companions -- his memento mori, for whom he would gladly have given up his life. Now he must do so for his child’s sake. The Doctor points out that the child’s birth was an accident which wiped out an entire species, but the child reminds him that this isn’t its fault. In any case, the Doctor is constantly putting his life in jeopardy for the sake of others, whereas the child is determined to hang onto its own life now that it has one. The TARDIS falls silent as the child urges the Doctor to make the choice he’s always wanted to, and give up his own life so that another may live.

The Doctor accepts that his experiences mean nothing if he can’t give them up for the sake of another who deserves life. This is why he sacrificed himself for Charley, and he now makes the same decision, agreeing to give up his life for the sake of the music. But the real Charley arrives and stops him from doing so. She is part of the Doctor now, and she will not allow him to give up his life -- not for her, and not for the child. As the music rages, Charley turns her back on her past life and gives her brooch to the Doctor, with all that symbolises. The child begs its parents one last time to give up their lives for it, but again, they refuse. The music dies away, squeaking a cry for help; the first words the Doctor and Charley heard from it are also the last.

The Doctor and Charley have out-evolved the music and survived, but the Doctor is prepared to go even further. In his current state, he already seems to have some inkling of the creatures they will encounter in this new universe, but why explore it when they can become it? Just by flexing his muscles, the Doctor shatters the confines of the glass tube, but Charley stops him from doing any more damage. She doesn’t want everything there is; she just wants herself and the Doctor, separate but together. The Doctor seems afraid to let go of her, but she promises that she’ll always be by his side. She and the Doctor thus push against each other, separating into their two individual bodies once more.