1856: The young Lees brothers of New Lanark, Scotland, buck tradition by swimming in a reputedly haunted stretch of the River Clyde near the falls at Corra Linn. The youngest brother, James, sees what he believes to be a gold coin glinting on the riverbed, but when he dives down to get it he fails to surface again. His brothers, John and Josiah, find him half-buried under rocks and pull him to the surface, but he fights them off, claiming to have seen their dead grandfather on the Other Side. James’ brothers take him to Dr Robert Kirkhope for treatment, but once they are alone, James shocks Kirkhope by speaking in the voice of Kirkhope’s late wife, Morag, and claiming that God took her life in childbirth to punish Kirkhope for conducting abortions for the village women. When James’ mother and father arrive, James speaks to his mother in the voices of the two unborn daughters Martha had aborted. Kirkhope and Martha manage to convince Mr Lees that James is raving, and Kirkhope decides to commit James to an asylum known as the Lock for “treatment”, thus ensuring his silence.

Nyssa has begun keeping a journal in the wake of Adric’s death and Tegan’s abrupt departure. The Doctor offers to cheer her up by taking her to the Great Exhibition of 1851 as promised, but the TARDIS alarms register a weakness in the space-time continuum -- and Adric’s ghost appears in the console room, claiming that he has been chosen as the Doctor’s spirit guide for the vital task that he must perform. Adric then vanishes, and the Doctor sets course for the weakness, intending to investigate. Despite her natural scepticism, Nyssa wonders if Adric really was a spirit from the afterlife -- and if so, is it possible she may be reunited with her father?

The TARDIS materialises in London, 14 February 1863. James Lees is now a ward of the Baroness von Luckner, and has been conducting successful séances for the crowned heads of Europe. The Baroness now presents James to Her Majesty Queen Victoria, claiming that James recently channelled the late royal consort, Prince Albert. According to James, Albert spoke of portals between this world and the afterlife, such as the one which James passed through as a child. Victoria agrees to hold a séance at the Mausoleum in Frogmore, where Albert’s body is kept; in the meantime, she despatches soldiers to Scotland to secure the portal at Corra Linn. General George Doulton, though unwilling to argue with his queen, does not trust the Baroness or James and thus sends a request to Scotland Yard, asking them to investigate the newcomers’ background.

The Queen and her entourage go to the Mausoleum, where, at James’ request, the Queen dismisses all but her closest advisors. James then enters a trance and, in the voice of Prince Albert, tells the Queen that it is possible for them to be reunited. The séance is interrupted by the sound of the TARDIS materialising, and when Doulton investigates, he finds the Doctor and Nyssa standing next to a “police box” and concludes that they’ve arrived in response to his request for help. The Doctor does not disabuse him of this notion, and Doulton thus introduces him to the Queen and to James Lees. When Nyssa meets James, he speaks to her in the voice of her late father, and she faints. The Baroness is furious, but James privately insists that this is not his fault; as he’d warned the Baroness before, he’s losing control of his powers.

The Queen offers to house the Doctor and Nyssa in Windsor Castle while Nyssa recovers, noting with approval the Doctor’s open mind on the subject of James’ powers. The Doctor believes that James is linked to the space-time weakness he detected earlier, but does he really have the power to speak with the dead? In any case, the Doctor soon determines that the “Baroness von Luckner” is an impostor, as does the Queen’s private secretary, Sir Henry Posonby. The Queen and the Doctor confront the so-called Baroness, and find her preparing to flee from Windsor Castle along with everything in her rooms that wasn’t nailed down. Caught out, she admits that she is a widowed actress named Sylvia Walker who sought a better life for herself by impersonating an aristocrat. However, she insists that James’ powers are real; she met him while on a tour of an asylum, and he spoke to her in the voice of her late husband. She thus adopted him and has been exploiting him ever since, using his séances as a distraction while she robs the grieving families. The Queen has Walker expelled from the palace in disgrace, but allows the Doctor to question James further and try to find out whether his powers are genuine.

In Scotland, Doulton’s soldiers arrive at the River Clyde and prepare to make camp by the falls. The men are under the command of the hard-bitten Sergeant Charles Vollmer and Lieutenant Reginald Ashe, an upper-crust young man who has obviously never seen combat. Vollmer dismisses the local claims that the area is haunted and places Privates Eric Hawthorne and Nicholas Johnson on first watch. However, that night Johnson sees the figures of his three brothers, who were reported dead in the Crimea, standing on the opposite side of the river, and Hawthorne must intervene to stop him from trying to swim across to them. When Vollmer arrives to investigate, there is no sign of the three figures.

Nyssa’s sleep is disturbed by dreams of a drowning child. When she wakes, the Doctor returns her journal, which she dropped when she fainted. While he is dealing with the faux Baroness, Nyssa returns to her rooms, only to find James standing on a balcony in the rain, blood trickling from his nose. Nyssa insists that he return to her room for a warm bath, and there, he tells her his story. In the Lock, James witnessed and was subjected to brutal “treatments” involving boiling mercury. He wants only to return to the peace he knew on the Other Side, but whenever he tried to escape he was caught and returned to the Lock. The longer he stays in this world, the weaker he becomes, and now he fears he is too weak to swim down to the riverbed and return to the portal.

James reluctantly agrees to let the Doctor to test his powers, and while the maidservant, Mary, keeps watch in the corridor outside, the Doctor fetches scanners from the TARDIS, sets them up around James and hypnotises him, regressing him to the moment when he crossed over to the Other Side. As James speaks of his encounter with his late grandfather and the sisters who never were, he begins to convulse, and the Doctor’s instruments go wild as an ectoplasmic figure rises from James’ body and advances on Nyssa. Playing a hunch, the Doctor tells Nyssa to picture her father, and the figure transforms into the image of Tremas. However, the manifestation is killing James, and Nyssa is forced to let her father go once more. The figure merges into James’ body again, and James passes out.

The commotion has attracted Sir Henry’s attention, and he forces his way past Mary to investigate. Nyssa fends him off, claiming to have woken from a bad dream, and though not entirely convinced, Sir Henry is embarrassed by Nyssa’s dishevelled state of near-undress and departs without pressing the matter. The Doctor spends the night collating the results of his experiment, but while he’s busy, James awakens and slips out of the room, no doubt intending to make his way back to the portal. The Doctor doesn’t notice him go until it’s too late; however, Sir Henry does see James slipping out of Nyssa’s bedchamber, and concludes that the newcomers are conspiring together.

The next morning, Nyssa awakens from further unpleasant dreams, and the Doctor informs her that James’ body is imbued with enough artron energy to kill a human being. Before the Doctor deals with this, however, he wants to speak to Nyssa about her feelings for her father, as he glanced through her journal before returning it and now believes that she’s suffering from post-traumatic stress. Nyssa denies this, but breaks down in tears when the Doctor points out that she barely reacted to Adric’s death on the outside, and finally admits to herself that on some level she blames the Doctor for failing to save her father or her planet. Before taking the results of his work to the Queen, the Doctor spends some time talking with Nyssa, helping her to heal some of her emotional scars at last.

The Doctor reports his findings to the Queen, but then learns that she’s sent troops to Corra Linn to investigate James’ story and has also arranged to send them a diving suit so they can reach the underwater portal. The Doctor reminds her that Heaven is not the only world on the Other Side, and the Queen, taking his point, appoints him her Scientific Advisor and orders Sir Henry to send a telegram to the soldiers advising them to do nothing until the Doctor arrives. He agrees to do so, but secretly holds off doing so until he’s received confirmation of the Doctor’s credentials from Scotland Yard. The Doctor and Nyssa take rooms at the Great Northern Hotel for the night while preparations are made for their departure, but General Doulton intends to accompany them; he feels that a military operation should not be placed in the hands of civilians, and if the portal does indeed lead to the afterlife, he intends to claim the new lands for the British Empire.

Naval lieutenant Peter Kempshall arrives at New Lanark with the diving suit, and while Vollmer, Hawthorne and Johnson escort him and the diving suit to the soldiers’ camp, the other soldiers construct a jetty out over the water. Lieutenant Ashe wanders off for a stroll along the river -- and is later found dead, a look of terror on his face and his hair bleached white. Kempshall sensibly allows Vollmer to take charge of his own men, and Vollmer opens Ashe’s sealed orders and learns that Kempshall is to use the diving suit to search for a portal to another world.

The Doctor, Nyssa and Doulton travel north via train, with the TARDIS in the baggage car. On the way, the Doctor explains to Nyssa that Victorian society is at a crossroads; as science overturns many deeply-cherished religious beliefs, the Victorians have become obsessed with understanding the nature of the afterlife. He visits the TARDIS, hoping to use its instruments to track down James, but while he is working he detects a disturbance in the dimensional rift. Obviously, the soldiers did not receive his instructions to leave the rift alone, and someone has found it and entered, causing the rift to split open further. The physical laws of the Universe are being overwritten, and the effect is spreading. The Doctor directs the power of the TARDIS towards the rift, imposing a temporary buffer between the two worlds, but the TARDIS is not prepared for the strain and nearly breaks up. The Doctor will not be able to do this again soon.

The disturbance was caused by Lieutenant Kempshall, who, per his orders, has donned the diving suit and descended into the river. On the surface, as Hawthorne and Johnson place Lieutenant Ashe’s body in a coffin, the dead man begins to twitch and babble deliriously about a woman named Daisy. When the Doctor establishes the temporary buffer, Ashe’s body falls dead once again, and Hawthorne and Johnson vow not to speak of the incident. Meanwhile, Vollmer becomes concerned about Kempshall’s long absence and orders his men to pull the lieutenant back to the surface, but when Kempshall returns, his hair has turned white. Kempshall claims to have seen his dead parents on the Other Side, and to have experienced an aura of great peace and contentment before some force expelled him back to this world. Presumably this force was the dimensional buffer the Doctor established, but Kempshall will never know this; the next morning, when Vollmer checks on him, he finds that Kempshall has died during the night, his body just like Ashe’s.

That night, Nyssa has a nightmare of a man who kills a woman named Daisy while trying to force her to say that she loves him. She does not know that she’s just dreamed of Lieutenant Ashe’s dark secret, but when she tells the Doctor about her dream, he theorises that the dimensional rift is affecting her latent psychic abilities. Doulton is joined by a second squadron of soldiers as the train approaches New Lanark, and the Doctor becomes concerned by the sizeable military presence -- particularly when he reaches the camp and learns that the soldiers have received no telegrams about his own arrival.

Back in London, Chief Inspector Lovesey of Scotland Yard visits Sir Henry and the Queen to inform them that the Doctor and Nyssa are impostors. Sir Henry fears that they are enemy agents, but the Queen points out that they had ample opportunity to assassinate her and did not. She concedes that Doulton must be warned that the Doctor and Nyssa are impostors, but is upset with Sir Henry for going behind her back, and thus leaves him behind in London while she and her advisors travel to Corra Linn to visit the portal.

The Doctor prepares to investigate the rift, using a portable force field generator to ensure that he does not damage it further. However, he is unable to dissuade Doulton from accompanying him in a second diving suit. The Doctor and Doulton pass through the rift together, but the Doctor finds himself alone on the Other Side in a rose garden. Adric arrives, claiming that he has been chosen to act as the Doctor’s spirit guide until the Doctor accepts the truth about his surroundings, but the Doctor notes several inconsistencies in Adric’s claims and hears Doulton screaming in the distance. The Doctor concludes that he’s still wearing his diving suit, even if he can’t perceive it, and when he tugs on the guide rope he knows is still attached to his wrist, the soldiers on the surface receive the signal and pull him and then Doulton back. As the Doctor retreats to the TARDIS to ponder his experiences, Doulton informs Vollmer that he intends to lead an expedition through the portal to colonise the afterlife. He thus orders Lance-Corporal Clarke to dam the river and make the portal accessible for the Queen’s arrival. Sir Henry’s telegram then arrives, and after reading it, Doulton orders Vollmer to place the Doctor and Nyssa under arrest.

In the TARDIS, the Doctor shows Nyssa a room full of mementos of his past companions, and claims that the Other Side felt just as real to him as this room -- which implies that it was assembled out of his memories. He also determines that the rift was damaged further when Doulton was pulled back out separately from the Doctor and thus without the protection of the Doctor’s force field generator. Whatever universe or dimension lies on the other side of the rift, its physical laws are beginning to assert themselves on this side, and the Doctor and Nyssa are experiencing some telepathic feedback as a result. That night, they both dream of a man being murdered -- and the next day, when Doulton’s men evacuate the residents of New Lanark so they can dam the river, they find that two of the villagers, like Ashe, have apparently died of fright. Though they don’t know it yet, James Lees has arrived -- and the dead villagers are Dr Robert Kirkhope and Martha Lees.

When the Doctor and Nyssa emerge from the TARDIS the next day, they are immediately placed under arrest. The Doctor tries to warn Doulton that the portal is too dangerous to allow the Queen near it, but Doulton threatens to have him executed on the spot if he interferes. Doulton’s character seems to have changed drastically, and the Doctor fears that he’s suffering from decompression sickness, or the bends. When Vollmer reports the deaths of the two villagers, Doulton accuses the Doctor and Nyssa of murder -- and when Vollmer points out that Ashe and Kempshall both died of similar causes before the Doctor and Nyssa had even arrived, Doulton tears a strip off him before the troops for daring to contradict a superior officer.

Clark and his men use explosives to create a temporary dam, which will last for three or four days only. While they wait for the gorge to drain, Doulton has his men prepare a diving bell for an expedition to the rift -- and despite the Doctor’s protests, he takes Nyssa as well, to ensure the Doctor’s good behaviour. While the Doctor is waiting for Doulton to return, he is contacted by James Lees, who remains desperate to cross back through the rift to the Other Side although the Doctor warns him that it’s too dangerous to try anything with the soldiers on guard. Doulton and his men then return without Nyssa, and when the Doctor demands to know what’s happened to her, Doulton has him gagged. Vollmer is becoming increasingly disturbed by the general’s behaviour, but is unwilling as yet to openly challenge his orders. Meanwhile, Johnson notes that Hawthorne has also changed since returning through the portal; for one thing, he seems much more willing to accept orders without complaint.

Nyssa awakens to find herself back in her childhood home on Traken, with both her father, Tremas, and her mother, Lucina. Lucina now tells the stricken Nyssa that she died in childbirth because of the newborn Nyssa flexing her latent psychic powers; however, she also claims that the Keeper foretold this, and that she chose to give her life for her child’s. Lucina insists that she bears her daughter no ill will, and urges her to use the same psychic powers to contact the Doctor one last time and say goodbye to her old life so she can join them fully on the Other Side.

As morning breaks at the British camp, Vollmer and Johnson see somebody apparently pushing a dead body over the pier into the river. The figure lunges towards them when they challenge him, and they open fire, only to find that they’ve just shot James Lees. Vollmer summons the Doctor, but he’s too late to save James -- and the “body” turns out to be a diving suit which James intended to use to reach the portal. Doulton finds the Doctor standing over James’ body, but as he demands an explanation, James’ blood rises up into the air, creating a window to the Other Side through which Nyssa speaks to the Doctor. The Doctor warns her not to accept what she sees on the Other Side, but Doulton shoots the pool of blood, shattering the “window” and sending the Doctor and Nyssa into shock. Doulton then charges the Doctor with the murder of James Lees, and announces that, after taking a second expeditionary force through the portal, he will return and have the Doctor executed. Vollmer tries to take responsibility for James’ death, but Doulton changes the charges to criminal negligence for failing to save his life, and threatens to charge Vollmer with treason if he tries to assist the Doctor in any way. Now certain that Doulton is no longer acting rationally, and fearing for the Queen’s safety, Vollmer disobeys orders and releases the Doctor, giving him the key to his manacles once Doulton and the second expeditionary force have gone.

Nyssa has forgotten her former life, and accepts that she’s been reunited with her father, mother, and little brother -- until it occurs to her that she was an only child. The young boy identifies himself as the real James Lees, who has hardly aged at all on this side of the rift. While Tremas and Lucina are elsewhere, James leads Nyssa to a tower where a dozen British soldiers are confined to a small cell. Tremas catches Nyssa looking in the tower, and explains that it is a prison for criminals stronger even than the Melkurs, which are frozen in place as soon as they touch foot on Traken’s soil. However, Tremas then finds that someone has left the cell door open and allowed the real General Doulton to escape. In a fit of rage, he kills Lance-Corporal Clark and threatens to kill the others, one by one, until they tell him where Doulton has gone. Hawthorne saves the others by stepping forward and telling Tremas that Doulton is heading back to the rift, and when Tremas kills Hawthorne, Nyssa finally realises that this can’t possibly be her father.

Queen Victoria and her entourage arrive at the camp to find that James Lees has been killed and the Doctor arrested and sentenced to death. Clark and Vollmer give conflicting reports, and the Queen doesn’t know what to make of them until Doulton arrives in a state of disarray, claiming to have been rescued from the Other Side by a young child named James Lees. The Doctor realises that the real Doulton never returned after all; the man they thought was Doulton was a doppelganger from the Other Side. However, the understanding comes too late, as the other soldiers from the first expedition turn on their fellows, seizing control of the camp and taking the Queen prisoner.

On the Other Side, the creature which has taken Tremas’ form finally admits that his people took images from Nyssa’s mind to shape her ideal afterlife; however, they have ceased trying to find a peaceful resolution to their problems with Earth. The rift first opened when a dead unborn child was cast through the membrane -- one of the aborted foetuses which Kirkhope had been burying in the swamp -- and ever since, the physical laws of Earth’s Universe have been affecting the Other Side. Its inhabitants tried to drive off the invaders from the other world by turning the memories of their own dead against them, resulting in the falls’ cursed reputation. When this failed, they sent one of their own as an ambassador in the form of James Lees, but he was tortured, abused, exploited and finally murdered, bringing the concept of true death to the Other Side for the very first time. Nyssa insists that the people of Earth meant no harm, but the damage has been done, and the beings on the Other Side now intend to invade and destroy Earth before the humans can do the same to them.

The doppelgangers in the British camp prepare to plant explosives around the rift, blowing it wide open -- an act which the Doctor realises will destroy the Earth, at the very least. Fortunately, the doppelgangers fail to search the Doctor, who still has the key to his manacles. He frees the others, but realises he doesn’t have time to both stop the doppelgangers and rescue the humans still trapped on the Other Side. Doulton thus volunteers to return, find his men and fight a delaying action while Nyssa and James escape. The Doctor distracts the doppelgangers and steals the explosives’ detonator, and while they’re chasing him, Doulton slips back through the rift, locates Nyssa and James and sends them to safety. Tremas and Doulton’s own double transform into monstrous creatures that attack the soldiers, and Doulton and his men prepare to fight, intending to hold the monsters back from the rift until the Queen can be transported to safety.

The Doctor frees Queen Victoria, but as he tries to take her to safety they are confronted by the animated corpse of James Lees’ doppelganger, who speaks in the voice of Prince Albert. The Queen faints, but the Doctor communicates with Lees and learns the full story. He realises too late that this is why the spirit of Adric appeared to him in the TARDIS; the beings on the Other Side were looking for someone to help them seal the rift. He offers to do so now, separating the two worlds before any more damage is caused, but it’s too late; the Other Side now knows death, and the beings intend to destroy the universe which has wrought these changes on their own. However, while the doppelgangers are distracted, Vollmer and his men blow up the temporary dam, and the gorge floods with water once more. Nyssa and James emerge from the rift, and with the help of a young sharpshooter, Private Benjamin Morrison, they escape steps ahead of the flood while the doppelgangers are swept away to their deaths.