A small blonde girl called Janna is haunting the castle. Twelve years old, she isn’t a ghost, just a girl left to scavenge and live off the good will of others. She, too, is haunted – by the ghost of her dead sister.

For a hundred years the man in the mirror has watched the lives of others but never sought to return, until now.

Janna watches two robots, Bill and Bott, carry the mirror to the Great Hall and fix it to the wall. The robots discuss how old the real mirror would be, but this is just a replica. After they depart Janna continues to watch from under the table as another man comes to inspect the mirror. Something about his reflection perplexes him. As he steps forward he realizes that his image in the mirror is holding a gun. The man in the mirror shoots him in the heart and then steps out into the room.

Janna races through a corridor pursued by the footsteps of her sister who calls her name. Janna shouts that she knows it should have been her who died and that she would change it if she could. A breeze disturbs her, impossible in the depths of the castle. It is accompanied by a rasping and wheezing until the TARDIS appears in an alcove and the Doctor steps out. He says hello to Janna, but she runs, pursued before her sister, before she can see the face of the woman who is with him.

Martha is disappointed. She has been promised the most brilliant theme park in this part of the cosmos but finds that she is in a damp and gloomy tunnel. The Doctor surmises that they have arrived too early and that the theme park has not opened yet, by which he means that the castle is still a frontier fort under siege from the Anthium or the Zerugma and the theme park has not been built yet. Martha wants guided tours and coffee shops not blood, death and danger. The Doctor decides to find someone to help them.

They encounter a sinister monk who appears to have taken a vow of silence but guides them to the main part of the castle. On the way they pass a variety of costumed figures: a soldier in plastic armour and a crocodile man in a rubber mask. They find themselves in the Great Hall where there are suits of armour, dark paintings and a shield over which two parallax rifles have been fixed. The mirror hangs at one end. The Doctor confirms that they are in Castle Extremis, before the signing of a peace treaty. They are met by High Minister Defron, the man who brokered the peace. The Doctor’s psychic paper leads him to think that they are observers from the Galactic Alliance, though he is surprised to see them announce themselves so openly.

Defron tells them that the Anthium and the Zerugma have been at peace for twenty years but are still officially at war until the treaty is signed. The castle, which the Galactic Alliance plans to turn into a theme park, has been chosen as the venue because of its strategic importance at the head of the Sarandon Passage. He leads them into a conference room where the delegates are seated and introduces them as GA observers. There are four delegates; an elderly lady, a middle-aged man and two crocodiles.

The old lady is Lady Casaubon, acting as the personal representative of the president of Anthium. Her opposite number is First Secretary Chekz, an ageing crocodile with full authority for Zerugia. The other crocodile is General Orlo, Supreme Commander of the Zerugian forces. It is he who provided the gift of the Mortal Mirror. Lady Casaubon’s aide is the citizen, Stellman. After brief introductions the Doctor and Martha are taken by Gonfer, the silent monk who they met earlier, to inspect their quarters. Once they step out into the open air Martha realizes that the castle is floating in space, surrounded by a semi-permeable bubble.

Making their way back inside they notice that much of the castle is still being rebuilt. The Doctor recounts its history as a monastery, home to the Mystic Mortal Monks of Moradinard. In one room they encounter Bill and Bott, the vaguely humanoid robots, rebuilding one of the walls. The Doctor, apparently at random, moves an old stone and finds a glass box, or rather, a book with glass pages. The monk, their guide, tries to snatch the book but fails. He turns and runs from the room. The Doctor notices the real Gonfer in the corridor, dressed only in his underwear, wondering who hit him and stole his robes.

The Doctor races out into the courtyard and towards the gatehouse beyond which he can see ornamental gardens. There is no sign of the monk but Janna is hiding in the shadows and she tells the Doctor that he went back into the castle. He introduces himself to her and she tells him that, ‘Today I’m called Janna,’ before skipping off down a gravel path through the garden.

The Doctor and Martha speculate on who would want to steal the glass book as they accompany Gonfer back to his rooms. He is embarrassed about being in his underwear but soon dresses in shirt and trousers. The Doctor asks for a tour of the gardens but Gonfer tells him it is a minefield, which was how Janna’s sister met her fate. Gonfer tells them that Janna and her twin, Tylda, were born in the castle. Where Janna was cheerful and always willing to help Tylda was surly and stubborn, enjoying insulting people. Their mother died soon after they were born and their father was killed in the reactor accident at Modolfin. The girls were left to fend for themselves in the castle and soon learned the safe paths through the garden. However, when Tylda upset one of the kitchen boys and he chased her she left the path and detonated a mine on the lawn.

Gonfer says that Janna has changed. Some days she is her old self, and sometimes she acts like Tylda, as if her dead twin was within her. As the Doctor and Gonfer head back through the castle Martha is distracted by a girl in the shadows. She says she hates Gonfer so Martha asks her which twin she is. None of them notice the cowled figure watching them.

In the Great Hall Lady Casaubon finds Professor Thorodin standing in front of the Mortal Mirror, apparently talking to himself. She invites him to join them for lunch and he leaves to get ready. As she turns to follow she does not see her reflection step out of the mirror but Stellman’s entry into the room causes it to step back, cursing. Lunch is rather informal. Martha is warned off the water truffles, which turn out to be fetid pond weed, but the Doctor helps himself to a plateful. The humans watch with horror or distaste and the Zerugians with interest. His first mouthful causes him to double up, but with pleasure, and he piles his plate with more. This leads him into an animated conversation with General Orlo so Martha takes the opportunity to join Stellman and Chekz as they try to negotiate privately on matters that Orlo would object to. Professor Thorodin arrives. An expert on antiquities, particularly the Mortal Mirror, he is keen to discuss the origin of the replica with Orlo. The general is deep in conversation with the Doctor (who is keeping secret his disgust for the pond weed). Orlo feels that the peace process is a lengthy form of surrender, possibly for both sides, but claims to believe that it is the best thing for his people. He says that his great grandfather destroyed the original mirror and his grandfather built the replica. When the Doctor sees Professor Thorodin nearby he rushes to him and produces the small glass book. Thorodin does not seem to recognize it or the language it is written in. The Doctor puts it away, remarking that it is probably in code and returns to Orlo.

Later there is a knock at Chekz’ door. A stranger is standing outside, apparently a man. He says he needs to see Orlo and tell him that the plan involving the mirror will not work because everything is so fragile. When it becomes apparent that Chekz does not know of this plan the man attempts to leave, saying that he is Sastrak and that there will be no peace or surrender. Hearing this name causes Chekz to grab the man’s arm, which shatters at the wrist and the man screams in pain before plunging the jagged end into Chekz.

His scream brings everyone running but he breathes his last before Martha can help him. She pulls a piece of glass from the wound in his chest. Defron thinks it must have been a grotesque accident that killed the reptile but the Doctor doubts it and Stellman announces that the weapons detectors are not set to look for glass. As they look down on the body a girl’s footsteps are heard running away from the room.

In the room General Orlo is keen to trace the cause of death. Martha presents the glass shard as self explanatory but Orlo says that was only the means. Was the death an accident, or suicide or murder? Stellman tells Martha that Orlo suspects him because his trade is assassin.

The Doctor pursues Janna into a corridor from which she abruptly vanishes. The robots Bill and Bott are there, hanging a picture. At first they cover for her but when the Doctor tells them she may have witnessed the murder of Chekz and that could be dangerous for her they tell her to come out. A section of wall swings back to reveal her tiny room. She invites the Doctor in. However, she remains uncommunicative, saying that she did not see the murder. But she admits to the Doctor that she is afraid of the man in the mirror and of her sister. The Doctor reassures her that her sister is dead. Janna agrees that she knows this but wants to know why she has come back.

As Martha leaves Chekz’ room she sees Stellman going back into it. She stands outside, listening to him search the room and decides to do the same to his. Unfortunately she is caught there by Stellman who produces a glass gun of his own. She dives for the door and races off down the corridor expecting to be shot at any moment. She finds Bill and Bott who guide her into Janna’s secret chamber. Martha tries to build some rapport with Janna by saying that she met Tylda but Janna says that was her before realising that Martha thinks she is crazy. Janna is perplexed by Gonfer, whom she used to taunt and made to hate her but who has become so friendly since Tylda’s death. More than this she avers that Tylda has literally returned, and is going to the many secret places that the twins used to share and living in them, leaving traces that Janna finds. At this point the secret door opens, revealing Stellman and his glass gun.

The Doctor is looking for Martha when he meets the monk who has been following him. He tells the Doctor to read the glass book and that it can be read if it is held up to the mirror. After the monk vanishes the Doctor takes the book to the Great Hall and begins to read: ‘I am the Man in the Mirror and this is my story.’ The book is a diary that the Doctor is finding increasingly interesting until he is joined by Defron. Defron tells him that he informed the GA about Chekz’ death and mentioned that there were already two GA observers present but the GA said that they had never heard of the Doctor and Martha. The Doctor refers back to an earlier conversation where Defron had come under the misapprehension that their names were Martha Mouse and Doctor Duck, citing this as an example of GA ineptitude. He implies that he and Martha are more than GA observers which leaves Defron mollified but still puzzled. The Doctor then changes the conversation to ask who Manfred Grieg was. It turns out that Grieg was a brilliant strategist who helped to recapture the Castle a century earlier. As a reward he was given the Mortal Mirror by Governor Pennard but it was a trap: manufactured by the Darksmiths of Karagula it reflects an identical but darker realm, possibly an alternative dimension, into which Grieg was lured. Defron surmises that the original mirror was smashed to prevent Grieg’s escape as he was the only one with the tactical acumen to overthrow the Third Occupation.

The man with the smashed wrist waits until the Doctor and Defron leave before reaching behind the frame to set the mechanism.

Defron and the Doctor discuss the death of Chekz and his appointment as a politician rather than a soldier at the negotiations. This leads Defron to discuss Orlo’s bloodthirsty reputation.

Janna is fleeing from the mocking laughter of her sister. She goes to hide in the Great Hall where her well-behaved twin shouldn’t follow since it is forbidden to them.

Stellman shows Martha that he is no longer carrying his gun. He asks her to accompany him back to his room where he shows her a glass hand, smashed at the wrist, which he found at the scene of the murder. They go down towards the hall where they encounter the Doctor. He looks at the hand but cannot decide on its importance. He sees Janna enter the Hall but does not give her away. Instead he returns it to Stellman before going into the Hall with Martha. He points out an anomaly in the mirror: it reflects the Doctor, Martha and the objects in the room, but not Janna as she hides under the table or runs out of the door, laughing.

They chase after her to find out if she knows why she has no reflection. The man with the smashed hand is hiding in the shadows. He sees that the Doctor has left the glass book on a table. He picks it up, adjusts a control behind the mirror and steps through the glass, which has not reflected him.

The Doctor and Martha split up. Martha finds Janna, but also hears her sister in the same corridor. She takes her back to the Great Hall where she confirms that Janna has no reflection. The Doctor enters with Gonfer. Martha sees both of them reflected in the mirror. She can also see the reflection of the glass diary, even though the object itself is not on the table in the Hall. Puzzled, she touches the mirror’s surface, which shimmers and allows her fingers to pass through. Despite the Doctor’s warning she steps through to the other side. She finds herself in a dimly lit version of the Great Hall. Looking backwards, she realises that there is no mirror here, merely an alcove with a solid stone wall. In the Great Hall the Doctor runs to the mirror but bounces off its surface with a clang.

He quickly realizes that the mirror is the original Mortal Mirror and is a portal to another dimension (which, he explains, is necessarily very small due to the amount of energy it would require to maintain). He finds the controls behind the frame and uses his sonic screwdriver to open the portal before realising that if anyone were to see an image or person crossing over it would cause irrepairable damage to the traveller. Janna says that she saw a man shot by his reflection and that the reflection is one of the important people in the castle. The Doctor tells Janna and Gonfer to leave the Hall and guard the doors while he goes to fetch Martha.

Meanwhile, Martha has crossed the darkened Hall and entered a dim corridor. She feels her way into a nearby room where she is horrified to discover a dormitory full of sleeping Zerugian soldiers. One is woken by a sudden flare of light and calls for Sastrak, asking if it is time. The source of the light turns out to be a match struck by the Doctor. He guides Martha back to the Hall where the mirror has reappeared in the alcove. She steps through to her own world, followed shortly by the Doctor, explaining that his delay was due to having only one shoelace and a detour to reclaim the glass diary.

Their reflections are behaving independently, striving to break through, driven by fear and anger. The Doctor says that they are dark reflections, distillations of the emotions that were felt on the other side. With his screwdriver he turns the portal into a mirror again. Outside the room Gonfer is alone, Janna having got bored and left. The doctor sends Martha to track her down while he finishes reading the diary to find out how Manfred Grieg was trapped in the mirror. Bill and Bott, replacing some stone steps, tell her that Janna went into the garden, possible to the maze.

When she reaches the garden Martha is puzzled: Janna can be seen briefly flitting through the hedges of the maze, but her laughter is audible closer at hand. She heads towards the maze. At the same moment the Doctor is reading an entry in the diary that seems to have been written on that very day, describing what Martha is doing. Martha has entered the maze and encountered a monk who shocks her by greeting her as a Time Traveller.

The Doctor reads how Martha flees from the robed monk. She catches a glimpse of Janna but the girl seems to be in two places at once and Martha wonders if the dead sister is in there with her. She loses herself ever deeper in the pathways, needing to find the girl if she is to get out again. The Doctor reaches the end of the last entry and slams the book shut, worried that Grieg’s style has shifted to an impersonal third person narrative and that Martha is in danger. As he runs through the corridors he bumps into Defron who tells him that the GA Press Corps are due and that he would like the Doctor to be there for the press conference. The Doctor agrees as he races off. He takes a short cut across the garden, using his screwdriver to trace mines that have not been dug up yet. When their clustering is too dense to avoid he detonates them.

Martha reaches the centre of the maze and finds a large chessboard motif with a giant statue of a Zerugian in the middle. Behind it a figure moves. It is the monk. He asks Martha if she has been through the mirror and read the diary. His hood falls back and reveals an old man’s face, apparently made of glass. Janna arrives and flings herself at him, knocking him to the floor. As he staggers away Martha asks the girl why she was running away but Janna says she wasn’t; she was following Martha to make sure the monk didn’t get her. Martha sees something glass on the floor and picks it up. She hands it to the girl to look at, but she has gone again, and it is the Doctor who takes it. Martha tells him she thinks that Janna’s sister is still alive.

As the Doctor leads Martha through the maze to the exit he ponders about Tylda’s apparent death, wondering if it ever really happened. They meet Defron who tells him that there is to be a ceremonial signing of the preliminary peace treaty, live on Galactic television. The Doctor asks who might know what happened to Tylda and is directed to Colonel Blench, commanding officer of the GA security force in the castle.

The most perturbing thing about their conversation with Colonel Blench, for Martha at least, is that he expects them, as GA representatives, to have the release codes which will allow his men’s weapons to fire. The Doctor, knowing that a force of Zerugians is sleeping in the mirror, merely ignores this detail and looks at the three still images which are the only record of Tylda’s death. These show the obviously dead girl in the gardens. Closer examination of her footprints shows that she was running, but a second set of prints suggest that she was being chased though there is no evidence of who her pursuer was.

They track down Gonfer in his rooms. All he can tell them is that Tylda used to annoy the staff and one day a kitchen boy ran after her and she died. He says he saw the whole thing and since then he has tried to be nice to Janna. They then go to the Hall where the Doctor attempts to switch off the mirror but they are attacked by Professor Thorodin, wielding a sword. Martha notices that the man has no reflection, and that one of his hands is missing. She also realizes that he is made entirely of glass. The Doctor parries the sword thrusts while trying to ask Thorodin questions but gets little more than anger in response. Eventually they have to run away but Thorodin chases and is set to deliver a fatal blow to the Doctor when Janna emerges and trips up the glass man, causing his face to crack. Stellman also appears and produces the glass gun. Two shots are enough to completely shatter Thorodin’s body. Stellman tells them that the gun was Thorodin’s and that he found it while searching the professor’s room.

Janna says that he is the man in the mirror that she saw killed by his own reflection. Stellman has trouble believing that a glass man emerged from a mirror but when they turn to ask Janna more she has vanished. The Doctor asks Stellman to delay the ceremony, or at least move it from the Great Hall. Martha mentions that Janna has no reflection but the Doctor contradicts her, saying that she followed the reflection into the maze earlier. However, in the corridor they meet the glass monk from the maze, and with her is the glass Janna, except, she tells them that Janna has been dead for years. The monk steps forward and introduces himself as Manfred Grieg, the Man in the Mirror.

First Janna tells them that she is Tylda. Janna died in the explosion and it was Tylda’s fault. She cannot understand why her sister runs away from her all the time. Then Grieg tells the doctor that the Mirror is a trap. He assumes that Orlo’s family preserved the original mirror, and then replaced it with the intention of hiding soldiers inside until they could spring out. The problem is, suggests the Doctor, that once someone has crossed over into the mirror they are made of light and if their reflection is seen they are fixed with a DNA made of a translucent matrix based on the silicon that makes the whole thing work. Grieg is unsure about the mechanics but insists that the mirror is a trap that allows no escape, even though a person can cross over from one side to another as often as they like.

He repeats the story of how he was tricked into the mirror by Pennard in an attempt to make him subservient. Pennard thought that a glass Grieg, afraid of his own fragile nature, would be easy to control and use for his own political ends. Grieg’s response was to hide in the depths of the mirror and refuse to co-operate. The trap, he points out, is that he can never be more than a slowly disintegrating glass man. Martha can see and hear his cracking glass features and the constant pain that he endures.

As they rush to the Hall Martha says they need to force the abandonment of the ceremony but the Doctor disagrees. The plan, and Orlo, must be exposed. Thus, when Stellman tells them that Defron would neither move nor postpone the ceremony, which is due to start, the Doctor is delighted. He finds Bill and Bott and asks if the sound system in the Hall has a good amplifier with tonal distortion built in. They agree. Then he whispers a plan to them. The Doctor and Martha confide in Stellman, letting him know that they are not really GA representatives. He takes his place on the dais and they find seats near the back of the Hall behind the throng of press, Anthium, Zerugian and others. The Hall is full. Defron has barely begun his welcoming speech when Orlo throws him from the platform when Orlo throws him from the platform and announces his plans for conquest. From the mirror emerge Orlo’s reflection and an army of Zerugian soldiers.

Orlo explains that the Zerugians are restarting hostilities by taking Castle Extremis again. Defron stands up to him, saying that Colonel Blench and his men will fight them off. Blench asks the Doctor for the codes to arm his men. The doctor walks to the front of the Hall, telling Orlo that the only way out of this is through peaceful negotiation but Orlo reveals that he knows the Doctor is not a GA representative and that none have been sent to the conference. The Doctor reminds him that an attack on Blench is a declaration of war on the whole Galactic Alliance. Orlo is still determined to fight it out and more troops emerge from the mirror, reflections of reflections. The Doctor finally has to give way and explain that the real GA observers have been present all along and are Bill and Bott. The two robots deliver the arming codes and the GA forces, now massively outnumbered, power up their weapons. The press corps dive for safety as the first shots are fired. The Doctor finds Janna hiding under a table and asks her to deliver his sonic screwdriver to Martha at the back of the Hall. Before she can take it a Zerugian points a gun at them and the doctor uses the screwdriver to shatter him.

The Doctor takes the opportunity of a lull in the fighting to offer Orlo a last chance to surrender but he is turned down. Janna reaches the robots who plug the sonic device into the sound system. A growing wave of sound paralyses the humans and causes the Zerugians to disintegrate into fragments. Before half of them are destroyed the glass Orlo reaches the screwdriver and rips it out, smashing it. He attacks Janna. Her scream causes him to shatter but not before a blow from his arm has knocked her off her feet. She bangs her head as she falls. Martha crawls to the unconscious girl, but can do nothing for now. The Doctor manages to switch off the mirror to prevent more reinforcements arriving but Orlo still has enough men to take the castle. He flings the Doctor away from the mirror to land near Stellman and Lady Casaubon.

At the back of the hall the GA troops have run out of ammunition and are fighting hand to hand, destroying Zerugians with chairs and defunct guns. Then the reflected Janna screams and her voice destroys the remaining glass reptile, though dealing severe internal cracking to her own glass body. Her first question is whether Tylda will be alright.

Orlo tries to take Lady Casaubon hostage but she stabs his hand with a shard of glass and Stellman kills him with a bullet from the glass gun. Lady Casaubon thanks the Doctor for all he has done and hopes that the Zerugians will send someone who will negotiate sensibly. Bill and Bott survey the wreckage of the Hall and bemoan the fact that they will be expected to tidy it all up.

Gonfer and Tylda approach the recovering “Janna’. The Doctor explains to Martha that the real Tylda annoyed Gonfer so much, when he was a kitchen boy, that he chased her into the garden. Unfortunately he was mistaken and chased the wrong girl. Janna was so terrified she ran into the minefield and died. Tylda’s reflection explains to Tylda that their sister is dead; she is not a ghost, just a reflection of herself. Grieg and Tylda return to the mirror after the two girls have embraced gently and Grieg has said farewell to the Time Travellers. Martha wants to know how he knew they were time travellers but the Doctor says it is obvious when she thinks about it.

A hundred years earlier the TARDIS materializes by the two robots as they replace a stone. The Doctor steps out and asks if he can hide a glass diary under the new stone to impress a girl in the future. The robots agree and in return he says he will put in a word for them with the GA, but they must give him the release codes when he asks for them. They ask what he means but he has already gone.