November 2004: Professor Harket has been camped out in the Norwegian forest for months, and at last he thinks he’s picked up the trail of the creature he’s been searching for. A girl is running scared through the woods as Harket pursues her; with luck he can get her back to Oslo in one piece, but he’s willing to shoot her if he has to...

Evelyn enters the TARDIS console room to find the Doctor in a particularly jolly and distracted mood. He eventually admits that he’s been conducting repairs to certain systems which haven’t been in use for some time -- systems available in every TARDIS which enable the Time Lords to hunt vampires. After much work, the Doctor has made a breakthrough with the Twilight virus, and he believes that he can finally cure young Cassie. Evelyn is delighted by the news, but it turns out that picking up Cassie won’t be as straightforward as the Doctor had hoped. He had intended to return just a few moments after he first dropped her off, but instead the TARDIS is trying to materialise in southern Norway some years later. The Doctor could override the co-ordinates, but he decides to follow the TARDIS’ intuition.

The TARDIS thus materialises in the Norwegian forest, and the Doctor and Evelyn step out into the brisk night forest air only to hear a distant gunshot. Evelyn rushes off to investigate, but it’s the Doctor who finds Professor Harket, overpowering him just as he’s about to fire a rifle at his fleeing target. Evelyn sees Cassie running away and calls out to her, but Cassie, distracted by the familiar voice, stumbles into one of Harket’s traps and ends up suspended from a tree. The Doctor wrestles Harket’s gun away from him and demands that he release their friend, but Harket, infuriated by the Doctor’s interference and judgemental attitude, informs them that Cassie isn’t his target -- and that the gun fires anaesthetic darts, not bullets.

Harket releases Cassie from his trap, but Cassie doesn’t seem particularly pleased to see the Doctor and Evelyn again. The Doctor demands an explanation from Harket, but before he receives one Cassie smells blood on the air. She leads them to the body, but Harket is puzzled when the hike covers nearly half a mile; how could Cassie have smelled blood from such a long way away? Evelyn is quite out of breath by the time they arrive at their destination, the petrified body of a young man coated with a bizarre blue slime unlike anything Cassie has ever smelled before. The Doctor advises Harket not to touch the slime, and points out marks on the dead man’s chest that look like handprints burned into the skin. Despite the gruesome discovery, Harket is delighted by the implications. The creatures he’s hunting do exist after all...

As Harket sets up camp, Cassie covers the body to protect it from scavengers, bitterly noting that she knows just how desperate scavengers can get. The Doctor promises to talk about it later, but in the meantime there’s a death to investigate. Harket settles in and explains that he’s been researching folk tales of the Huldra, a legendary troll with the body of a beautiful woman and the tail of a cow. According to legend, the Huldra lures men into its grasp with its beautiful singing voice and then drains the life out of them. The Doctor dismisses this story as merely another variant on the tales of sirens found in so many civilisations, but Harket is convinced that the legends have a basis in fact and believes that the young man they’ve found was killed by a Huldra. He thus leaves the Doctor, Cassie and Evelyn by the campfire while he searches for a spot with clear reception so he can call his university and report his find.

Elsewhere, the computer system Oracle receives a top-priority encrypted message. The deputy director is unavailable, and Sergeant Frith, head of security, therefore takes the message, using a voice-print system to confirm his identity and clearance. He passes the message on to Dr. Crumpton, who is delighted by the news and reports to the deputy director that field agent Artemis has at last made contact with “Lazarus”. Frith organises two extraction teams; the deputy director will meet them at the rendezvous point, while Crumpton remains behind to set up work on Project: Lazarus...

Evelyn is still recovering from her exertions, but she assures the worried Doctor that she’ll be all right. However, they then now deal with the bitter Cassie, who has been through hell ever since the Doctor dropped her off. Nimrod survived the explosion at the Dusk casino, and she’s been on the run from Forge agents for years, barely surviving by the skin of her teeth, and always desperately waiting and praying for the Doctor to return. The Doctor and Evelyn are upset and apologetic, and the Doctor assures Cassie that her ordeal is almost over. But it seems that Cassie would prefer to stay and investigate the young man’s death before returning to the TARDIS, and the Doctor concedes that he too is intrigued.

Soon afterwards, the TARDIS is being carried by one helicopter and the Doctor and Evelyn are aboard another, on their way to the Forge with Nimrod and the hostile Cassie watching over them. The Forge is located on Dartmoor beneath the ruins of an abandoned asylum, and Frith and Crumpton are waiting for them to arrive. Crumpton has been eagerly anticipating her first meeting with the Doctor, but Nimrod orders her to begin a preliminary examination of the captive alien first. The Doctor learns that Nimrod is now the deputy director of the Forge, but is surprised when Nimrod claims that he and Evelyn are not prisoners; they’ve just been brought here for a debriefing about their encounter with the alien. He apologises for their rough handling and for Cassie’s “overzealousness” in the matter of Professor Harket, and assures them that they are free to leave at any time; however, he first invites the Doctor to tour of the Forge. The Doctor doesn’t particularly trust Nimrod, but concedes to an interest in what happens at this facility and agrees to accompany him on the tour. Evelyn isn’t feeling well, and Nimrod thus orders Cassie to stay with her. Cassie reluctantly obeys orders, but is upset about being given babysitting duties and by Nimrod’s refusal to support her killing Harket.

The alien is taken to Crumpton’s laboratory, where Crumpton notes Frith’s revulsion at the sight of it; he doesn’t enjoy working with alien life forms, but has little choice. No one ever leaves the Forge voluntarily. Crumpton sets to work, but first orders Oracle to download all files relating to the figure code-named “Lazarus.”

The Doctor seeks reassurances that the alien won’t be harmed, and Nimrod -- who decides to call the alien a Huldran, after the late Professor Harket’s research -- assures him that he only wants to study the effect of the alien venom in order to synthesize a stun weapon. A secondary team has found the remains of its spacecraft, and once the Forge scientists have examined it they will try to help the Huldran return home. The Doctor remains sceptical, but Nimrod assures him that the Forge has learned from its past mistakes and remains committed to its mission to improve mankind’s lot through the study of alien technology. He also claims that Cassie came to work for the Forge willingly after he tracked her down via rumours on the Internet; after he trained her, she became the Forge’s primary field agent in northern Europe, and was given the code name “Artemis” after the Olympian goddess of hunting.

Evelyn rests in the mess hall and speaks with Cassie, who blames Evelyn and the Doctor for abandoning her in the middle of nowhere and leaving her to scavenge for herself. Cassie lived in the dark and cold for months, desperately trying to evade Forge agents and stop herself from feeding on human blood; despairing, she tried to commit suicide many times, but she healed too quickly for normal methods to kill her and couldn’t stake herself in the heart after having seen the results once before. When Nimrod finally tracked her down she accepted his offer of employment, and claims that working for the Forge has given her a sense of purpose and family at last. When Evelyn reminds her that she already had a family, she’s surprised by Cassie’s violent denial. Furious when Evelyn keeps insisting that she had a son named Tommy, Cassie threatens to drink human blood for the first time, and then cruelly reveals that she can hear Evelyn’s heart beating -- and knows the real reason why Evelyn is so out of condition. Evelyn confesses that she had a mild heart attack just before she met the Doctor, which is one reason why her university was trying to get her to retire -- but just when it seemed that her life was winding down, the Doctor came along, offering her something new and exciting. If he learns of her heart condition, Evelyn knows that it will end their travels together -- and she fears that this new, brutal Cassie might tell him.

Nimrod leads the Doctor to the Forge’s main archive, and Oracle grants them access once Nimrod confirms his identity via voice-print. Inside, the Doctor is appalled to find rows of dead alien life forms, artefacts and technology from some of the most dangerous races in the Universe. Nimrod assures the appalled Doctor that in the worst case, should the archive be breached, the fail-safe Hades Protocol will sterilise the entire Forge complex. He now drops all pretence at friendliness and reveals the real reason why the Doctor is here -- their archive is lacking in data on the Time Lords. Nimrod and Frith force the protesting Doctor into Crumpton’s laboratory, where he is strapped down to something like an operating table and Crumpton takes a blood sample for analysis. Following their previous encounter, Nimrod collected all of the information he could on the Doctor, and became fascinated by his ability to regenerate, an ability far beyond Nimrod’s own rapid healing. Now that the Doctor is their captive, Crumpton and Nimrod intend to torture him to death in order to induce a regeneration and see just how the Doctor “rises from the dead”. Project: Lazarus is now on the clock. The Doctor protests to no avail, and Nimrod and Crumpton begin to administer electrical shocks, increasing the voltage as he screams...

Evelyn, upset by Cassie’s cruel taunts, insists that her association with Nimrod has changed her for the worse -- and that whatever Cassie may say, Evelyn knows that she has a son. She’s seen the pictures. Cassie tries to block out what Evelyn is saying to her, but Evelyn realises that she’s parroting phrases that have been burned into her memory and urges Cassie to fight past them. Cassie is unable to resist her pleas, and the blocks on her memory come down. Just as Evelyn feared, Cassie did not come to the Forge willingly; she was brainwashed by Nimrod into forgetting all about her son and believing that the Forge was her only family. Cassie collapses in tears, realising what a monster she’s become and understanding that Nimrod was only using her all along as bait to get the Doctor.

The Doctor has been tortured nearly to the point of regeneration when Cassie bursts into the laboratory, knocks Frith clear across the room and overpowers Nimrod. Her voice-print authorisation enables her to order Oracle to release the Doctor, but as they flee and are reunited with the waiting Evelyn, bulkheads slam shut around them and Cassie finds that Nimrod has revoked her security access. She still has the strength of a vampire, however, and she rips the control plate from the bulkhead’s security lock, enabling the Doctor to hotwire it open. Having to do so for each bulkhead delays them, however, and by the time they reach the storage bay where the TARDIS is being kept, Nimrod is waiting for them. Cassie holds Nimrod off while the Doctor gets Evelyn to safety in the TARDIS, and overpowers him -- but the moment she turns her back on him, he pulls out his nanobot-crossbow and shoots her through the heart. Despite her agony, Cassie begs the Doctor to take care of Evelyn, but the Doctor can do nothing for Cassie as her blood begins to boil and her body literally explodes.

Many years have passed for the Doctor since his escape from the Forge, and he is now in his seventh incarnation. As he practices the piano, the TARDIS is shaken by a wave of temporal energy, and though it stops as strangely as it began, the Doctor is able to trace its source -- to a most surprising location. Soon the TARDIS has materialised on Dartmoor, and the Doctor has returned to a place he’d never expected to see again...

The Forge is under attack. Dr. Crumpton’s quick thinking has saved the day on several occasions, but sooner or later their luck will run out and the Forge will be overrun; however, Nimrod still berates Crumpton for cutting the experiment short, despite the risk that the defences could have fallen and everyone in the Forge could have been killed. Crumpton notes some strange readings during the most recent assault, which suggest that something solid materialised briefly and then vanished again. When proximity alarms indicate the presence of an intruder in the grounds above, Nimrod initially passes off the problem onto Frith, but he quickly becomes interested when he sees a video image of the intruder -- and Oracle matches his appearance to incidents in Shoreditch, 1963 and Carbury, 1997. Lazarus has returned...

Nimrod sends up a lift for the Doctor, who accepts the offered ride down into the Forge. Frith and his men are waiting for him, and Frith, irritated by the Doctor’s flippancy, comments that he prefers the other one. He brings the Doctor before Nimrod, and the Doctor greets his host coldly, recalling Cassie’s fate only too well; however, he has no choice but to work with Nimrod to solve the disruptions in the Time Vortex. Nimrod admits that the Forge’s scientists obviously need help, and the Doctor reluctantly agrees to assist their experiments in order to prevent any further damage. When he is shown the telemetry data, he scoffs at the work of the scientist who’s come up with such ridiculous results -- and is startled when the Sixth Doctor strides in, having taken great offence to that comment...

Nimrod reveals that the Sixth Doctor has been assisting them, but this comes as a great surprise to the Seventh, who has no memory of these events and can’t believe that he ever would have joined the Forge willingly. He reluctantly accompanies his former self to Crumpton’s laboratory, where she’s been running an analysis of the last attack and recharging their defence systems. The Sixth Doctor mocks the Seventh’s demands for an explanation, mimicking his voice perfectly as he does so, but eventually stops being childish and explains that the attack centres upon the wreckage of the Huldran ship from Norway. The Seventh Doctor is disgusted to learn that the Forge scientists did indeed cannibalise the ship and kill the Huldran while experimenting upon it. Evelyn would never forgive the Sixth Doctor for working with Nimrod after all of these, especially after what happened to Cassie. She never did.

The Sixth Doctor requests a private word with the Seventh, and once they’re alone he explains the situation more fully. The Earth is under attack by the Huldrans, whom it seems want revenge for the murder of the Forge’s captive. The Sixth Doctor offered his services to help prevent an invasion of Earth, but to prove his good intentions he had to let Nimrod and Crumpton aboard his TARDIS. It soon became clear that they wanted to study its technology, and to prevent this the Sixth Doctor removed a vital component and was left with only a shell of a police box. He’s thus stuck here in the Forge, unless he and the Seventh Doctor escape in the functioning TARDIS outside. The Seventh Doctor still doesn’t remember any of these events, and is puzzled by the Sixth Doctor’s apparent desire to escape before solving the Huldran problem. He thus agrees to remain and help deal with the Huldrans -- but by reasoning with them, rather than killing them.

Nimrod ushers the Doctors back into Crumpton’s laboratory, where she explains their plan. The Huldran ships don’t actually travel of their own accord; rather, they carry portals which allow the ships to travel through a dimensional vortex. The disturbance which the Seventh Doctor encountered was the result of the attackers trying to breach the portal which the Forge salvaged from the ship that crashed in Norway. Nimrod refuses to shut down the portal while there’s still a chance they can utilise its technology, but if they get a sample of the TARDIS’ impervious exo-shell and lace that material into the portal cover, that should be enough to keep the Huldrans out for good. The Seventh Doctor questions Nimrod’s ethics, but when the Sixth mockingly mimics his voice again -- he can do a good imitation of Nimrod, too -- the Seventh, though irritated, agrees to help. He will do so on his own terms, however, and hopefully in the process he can shut down the Forge for good.

As the Seventh Doctor considers his next move, Nimrod and the Sixth privately discuss their own plans; once the Huldran problem is solved and they have access to the TARDIS, they intend to dispose of the Seventh Doctor. However, the Seventh Doctor has come to certain conclusions. The Sixth Doctor’s TARDIS may no longer be functioning as a time machine, but why can’t they take a sample from its exo-shell if they still have the exterior? As he’d expected, the Sixth Doctor is unable to answer this question, and the Seventh thus heads off to speak to Crumpton, leaving Nimrod with the fuming Sixth Doctor -- or whoever he really is...

The Seventh Doctor intervenes as the Huldrans slash away at his sixth self, and somehow he convinces them to calm down and lower their weapons. However, before he can communicate further, Crumpton shuts down the portal, and Frith’s security team surrounds the Huldrans, taking them prisoner. This was not the Doctor’s intention, but Nimrod ignores his protests and orders Crumpton to take the Huldrans to the holding cells. In the meantime, the grievously injured Sixth Doctor is taken to the sickbay -- but his arm has been severed, and the medics are unable to reattach it, confirming the Seventh Doctor’s suspicion that all is not as it seems. The Doctor also notes that Frith doesn’t really like his job, but Frith explains that he was used as a scapegoat in an military cover-up and was reassigned here against his will. He prepares to lock up the Seventh Doctor on Nimrod’s orders, but the Doctor surprises him and knocks him out, intending to speak to his sixth self privately and get some real answers.

Nimrod has ordered Crumpton to resume the aborted Project: Lazarus and prepare to dissect the Huldrans, but for perhaps the first time ever, Crumpton is questioning the ethics of her work for the Forge. Nimrod scoffs when she threatens to go over his head, reminding her that he holds the real power in the Forge, not the director. It seems Crumpton has little choice but to obey, but her objections are interrupted by Oracle, which has detected an unidentifiable energy spike. And it’s not coming from the portal room...

The Seventh Doctor wakes his injured sixth self, who is horrified to see that he’s lost his arm -- but the Seventh Doctor is more intrigued by the fact that this grievous physical trauma didn’t trigger a regeneration. Despite the Sixth Doctor’s protests, the Seventh makes mental contact with him. The weakened Sixth Doctor is unable to resist as the Seventh pushes past his blocks and looks over his memories. Some, of torture at the hands of Nimrod and Crumpton, are familiar. Others are not -- and when the Seventh Doctor has finished viewing them the truth is clear. The “Sixth Doctor” is not the Doctor at all, but a clone created from the blood sample which Crumpton took the last time the real Doctor was at the Forge.

The clone admits that three clones were created from the Doctor’s DNA; the first died within seconds of birth, and Nimrod killed the second, giving it a few days to stabilise and then slitting its throat to see whether it would regenerate. It didn’t, but the surviving clone demonstrated some of the original Doctor’s personality traits and natural abilities, and Nimrod decided to let him live on as the Forge’s scientific advisor. Unfortunately, the cloning process was unstable, and though Crumpton has periodically stabilised the clone’s genetic structure, it seems that this last attack was too much for him to handle, and his degeneration is accelerating. But the Doctor suspects that there’s another reason for the clone’s rapid deterioration, and the clone realises that the Doctor saw something else during their telepathic contact -- something which the clone hadn’t allowed himself to remember. Horrified by the memories which his communion with the Doctor has stirred up, the clone pulls himself up off the operating table and sets off to investigate.

Crumpton is astonished by the data Oracle has collected, data which seems to indicate that someone in the Forge is capable of reading thoughts. Nimrod deduces that the Doctor and his clone must be capable of sharing thoughts when brought into close proximity, and Crumpton then notices that the Huldrans are also reacting to the psychic stimulus. The entire horde is moving in unison, which implies that they’re a telepathic gestalt -- which explains why they’ve been attacking the Forge so ferociously, as the death of the original captive must have sent a shockwave of pain through their group mind. Crumpton sees this as an opportunity to communicate with an alien intelligence, but Nimrod has lost interest in them now, since they have proven themselves too vulnerable to be of use. He orders that they all be killed, but Crumpton refuses, and Nimrod finally tires of her disobedience and shoots her with his crossbow as she attempts to communicate with the Huldrans. The results are not as gruesome as they were with vampires, but are just as fatal.

The clone leads the Doctor into the room he saw in his memories, the same holding bay where Cassie was murdered, and the home of Project: Lazarus. The Doctor still recalls Cassie’s security codes, but the bay has changed since the last time he saw it. To the clone’s horror, the room is filled with dozens of mutated versions of the Sixth Doctor, all of them dying and pleading for release. The real Doctor finds notes which confirm his suspicions; the clone is not three years old as he had believed, but only days old. The cloning process is even more unstable than he believed, and for each “successful” clone ten failures are produced and kept here for tissue experimentation. Each “successful” clone lives only for a few days, and Crumpton then takes another DNA sample and creates yet another clone with the memories of the previous one.

The clone is shocked by this discovery, and the cries of the other clones echo in his mind, driving him mad with rage. The true evil of the Forge exposed, the clone orders Oracle to activate the Hades Protocol, mimicking Nimrod’s voice, giving the security codes to authorise the total self-destruction of the Forge and ensuring that the command cannot be overridden. The Doctor pleads with him to spare the innocents trapped in the Forge to no avail, but the clone does give him six minutes to rescue the Huldrans before the Forge is destroyed.

Alarms sound throughout the Forge, and as the lower levels begin to explode, Frith tries to organize a full evacuation. The Doctor tracks him down and asks him to help rescue the Huldrans; the other Huldrans on the other side of the portal could regard the deaths of the Forge’s captives as a declaration of war. The furious Nimrod then confronts the Doctor, blames him for destroying Nimrod’s life’s work, and evacuates, ordering Frith to kill the Doctor and ensure that all of the artefacts in the archive room are taken to safety. Frith, realising that Nimrod has just walked out and left him to die, instead throws in his lot with the Doctor.

Before leaving the Forge, Nimrod tracks down the clone Doctor, who is still in the Lazarus laboratory, surrounded by the babbling, rejected experiments. Nimrod realises that the clone’s latent telepathy is picking up the cries of his other selves, and that the pain is driving him mad. The clone is enraged by what he’s learned, but is too weak to attack Nimrod -- who reveals that he’s lost count of how many clones there have been since Project: Lazarus began. As far as Nimrod is concerned, the clone standing before him is just another failed experiment, and Nimrod shoots him down before leaving.