6 August, 6048. The Doctor and Ace catch a shuttle to the planet Heritage, where the Doctor plans to visit his old friends the Heyworths. Heritage is a dusty two-village mining colony which has been dying slowly ever since scientists learned how to synthesize the formerly rare mineral thydonium. But when the Doctor and Ace arrive, they realise that the colonists’ spirit is already long dead. Within minutes of their arrival, they are advised to leave by the town Sheriff. When the Doctor reveals that he’s here to visit the Heyworths, Sheriff has little choice but to report the strangers’ arrival to the man named Wakeling, who sends his thugs Ed, Christa and Bernard to investigate further. Hidden away in Wakeling’s house, a young girl named Sweetness watches it all, curious but uninvolved.

Ace is disturbed by the Doctor’s strange behaviour; for weeks he’s avoided going anywhere interesting, and he actually seems to be taking Sheriff’s warning seriously. Cole, a barman with a cybernetic arm, reacts strangely when Ace reveals they’re here to visit the Heyworths, and tries obliquely to warn her off. Even then, the Doctor refuses to take an interest. Ace thus takes matters into her own hands, and visits Lee, a friendly but reserved man who may be willing to lend them his horses for the trip to the Adobe Flats. However, Lee’s horses have vanished from his paddock, and Ace realises from Lee’s reaction that this isn’t the first time something like this has happened. Lee is reluctant to go into details, but she deduces from his cryptic comments that something on Heritage has caused things to disappear -- things and people, including Lee’s lover Ryan.

The Doctor is confronted by Wakeling’s three thugs: young lovers Ed and Christa, and Bernard, a cigarette-smoking human-wannabe Cetacean who requires a mechanical walker with a water tank to survive on Heritage. They are here to reinforce Sheriff’s none-too-friendly warning, but the Doctor ignores their unsubtle threats and instead books a room with Cole. Sheriff follows the Doctor to his room, where he finds that the Fussies -- the colony’s ubiquitous, small, self-motivated cleaning robots -- seem to have taken a shine to the Doctor for some reason. Once again, Sheriff tries to warn the Doctor off, but the Doctor informs him that he has no interest in digging up the colony’s secrets; he just came to visit his friends, and now all he wants to do is leave, before it’s too late.

The Doctor sets off to find Ace, but is stopped by Wakeling, who wants to see the stranger for himself. Realising that Wakeling is toying with him, the Doctor accepts his invitation to stop in for a cup of tea. Wakeling shows off his pet raven, a three-year-old clone, and the Doctor acknowledges that it’s a scientific marvel for this era -- but he also notes that the child’s toys lying scattered about the house seem too perfectly positioned, as if the scene has been staged. Wakeling lets the Doctor go on his way, believing that he has no intention of digging up the past, but Bernard isn’t so sure. Sheriff has contacted the outer planets and learned that nobody has heard of the Doctor or Ace; if necessary, it would be the work of a moment to make them “disappear” as well.

Ace meets the Doctor and warns him that something’s terribly wrong here -- and is shocked when the Doctor agrees and tells her that they must leave immediately. Furious with him for not being the crusader she knows, Ace sets off to investigate on her own. The Doctor is unable to stop her, and realises that to protect her he must remove the need for her to investigate. He thus asks Cole to make an off-planet call, but Cole struggles with his conscience, loses, and tells him this isn’t possible. The Doctor retreats to his room, and Sheriff starts downing shots of Cole’s home-distilled vodka, knowing that the Doctor is as good as dead now. Bernard bursts into the Doctor’s room and strafes it with bullets, but the Doctor is sitting next to the door and remains unscathed. He demands that Bernard tell him what really happened here, but Bernard refuses to do so. The Doctor storms out, and Sheriff lets him go, despite the risk of what will happen to him if the Doctor learns the truth.

Ace concludes that the monsters are hiding out in the mines, and asks Lee for help. He tries to warn her off, insisting that there are no monsters here and that Ryan died accidentally in a cave-in. Ace doesn’t believe him, and sets off, sure that she’ll be able to stop the monsters as she always does. Lee reluctantly goes to Wakeling and warns him, and Wakeling sends his thugs out after her. Ed and Christa ride out on the horses which they “borrowed” from Lee without asking, and Lee retreats to his home, wracked with guilt by this fresh betrayal.

Crossing the Adobe Flats in search of the mine entrance, Ace instead finds a single landmark -- the burnt-out shell of an old farmhouse, with a single grave beneath an alien tree. Inside the house, she finds the Doctor staring at a single surviving photograph of the Heyworths... and Ace finally learns that Mrs Heyworth was once known as Melanie Bush. For the first time, Ace really understands that she too could die and that the Doctor would be unable to stop it. The Doctor insists that as time travellers, they have the option of leaving now without learning too much, allowing for the possibility of Mel’s survival rather than burning these events into the timeline; however, Ace insists that if this was her, she’d want the Doctor to find out who did this, and make them pay for it.

Bernard, Ed and Christa arrive at the house, and the Doctor tries to lure them away from Ace; however, the ground gives way beneath his feet, dropping him into one of the abandoned mines. He is saved by a colony of Fussies, who carry him to a miniature city constructed out of bits and pieces they have scavenged from the human colony. There, the Doctor finds that the Fussies have been “reproducing,” building their own baby Fussies out of spare parts. He also finds a blueprint for the Fussies, covered with annotations in Mel’s handwriting, and realises that she has augmented them, which is presumably why they recognise and are protective of him. But the Fussies also show him something far more disturbing -- a medical tool caked in dried blood. The murder weapon...

Wakeling’s thugs drag Ace back to his home to explain herself, but by this time Lee has had second thoughts, and he rescues her, using dynamite from the mines to distract the others. Ace uses her own nitro-nine to create an even bigger “distraction,” and she and Lee head for the mines to rescue the Doctor. The Doctor is waiting for them, but despite all they’ve seen, he still refuses to get involved directly, planning instead to call in the Adjudicators. However, Lee reveals that only Cole can make an off-world call, and to get to Cole, the Doctor will have to get past Bernard. As a backup plan, the Doctor sends Ace and Lee into the mines, telling them how to wire together bits and pieces of the Fussy city to create a radio transmitter.

The Doctor delivers the murder weapon to Sheriff, who has seen it before and doesn’t need the Doctor to point out that it’s a standard medical tool used by geneticists. Now Sheriff must decide once again what to do about it. The Doctor retreats to Cole’s bar, where Cole has set up an interplanetary conference call for Wakeling rather than contacting the Adjudicators as he wants to. The Doctor and Cole are surprised when Sweetness arrives; she normally does what her Daddy tells her, but today, interesting things have been happening and a Fussy has opened up her bedroom window, giving her the opportunity to sneak out and investigate. The girl remains silent, but Cole knows that she has good reason not to speak to anyone in Heritage. Cole is unnerved when the Fussies cluster around the Doctor and Sweetness, and shoos them away -- but the Doctor looks into Sweetness’ eyes, and guesses what they were trying to tell him.

Ace and Lee make their way through the mines, wary of the crumbling roof and of the remaining sticks of dynamite Lee is carrying. They eventually reach the Fussy city, where Ace follows the Doctor’s instructions and wires the “buildings” together into a makeshift radio with which she contacts her shuttle pilot. She calls him back for a pick-up, but the radio shorts out before she can tell him to contact the Adjudicators. Lee, who was supposed to be watching the connections, is instead staring at a Fussy “house” constructed from Ryan’s bones. This was no accident, but it wasn’t the Fussies that killed him; Ryan couldn’t live with himself after what happened. Wracked with guilt, Lee finally admits to Ace that the monsters on Heritage are all too human...

Wakeling is enraged when he finds Sweetness missing, and sends Bernard to fetch her before the conference. Convinced that the Doctor kidnapped her, Bernard goes straight to Cole’s, where he finds the Doctor and Sweetness together. He forces them back to Wakeling’s, where Wakeling tells the Doctor that it doesn’t matter what he does; this video conference will give Heritage back its future and secure Wakeling’s place in history. But the Doctor knows that all history cares about is the bodies in its wake -- and he also knows that Sweetness is the Heyworths’ daughter, and much more besides. The Doctor allows Bernard to march him back to the bar, but when they arrive, Cole finally takes responsibility for his part in the tragedy and attacks Bernard, knocking him out. The Doctor disconnects Bernard’s translator so he can’t tell anyone what happened, and as he and Cole set off for the shuttle pad, Cole finally tells his story...

The truth comes out at last. One night, Mel argued with Wakeling, who struck her down, killing her before her husband’s eyes. Cole happened to be there at the time, and he summoned Sheriff -- but Heritage’s future had already been taken away, and the people were relying on Wakeling to make the colony known again. Sheriff, Cole and Wakeling thus agreed to remain silent and let Wakeling continue his work. Ben Heyworth remained silent through shock, but only until he found the murder weapon which Sheriff had “lost.” Armed with this evidence, Ben asked Cole to call the Adjudicators, but instead, Cole went to Sheriff, who went to Wakeling. Wakeling thus addressed the entire town and confessed the truth -- and a town full of people who’d seen their livelihoods taken away were given a choice between letting their colony die slowly or letting Wakeling finish his work and make Heritage known across the galaxy. That night, Heritage died within, when the mob walked across the Adobe Flats, tore Ben Heyworth apart with their bare hands and burned down his house.

Shaken by Lee’s revelations, Ace lets the Fussies guide her away while he stays next to Ryan’s body, wracked with guilt... and holding one last stick of dynamite. The Fussies guide her back to the shuttle, but Ed has beaten them all to it and murdered the pilot, an eagle-like alien named Hadoke. He also sabotages the shuttle, and Ace arrives just in time to see it explode. The Doctor and Cole escape, but Ace is infuriated with the Doctor for letting things go so far -- and this time he agrees with her. He knows that something terrible is coming, and, fearing that he won’t be able to handle it, he came to Heritage to say goodbye to Mel, hoping to set his past to rights and let another Doctor deal with the future. Instead, he’s found a reminder that there is evil in the Universe... and that it’s his job to stand and fight it.

Wakeling prepares for the conference, ignoring Bernard’s futile efforts to squeak a warning to him about the Doctor. Outside, Cole fights off Ed, allowing the Doctor to walk into Wakeling’s sitting room just as the conference begins and accuse him of murder before all the assembled delegates. While Wakeling is distracted, Ace sneaks into the house and takes Sweetness away. Sheriff then bursts in, thoroughly drunk and guilt-ridden; he has thrown away the murder weapon at the Heyworths’ farm again, but now has returned to arrest Wakeling as he should have done so long ago. Bernard, unable to understand what is being said, misinterprets Wakeling’s body language entirely and guns Sheriff down in front of everybody. The horrified delegates sign off, leaving Wakeling alone, his triumph in ashes.

Furious, Wakeling turns on the Doctor, informing him that Mel came to him in the first place. A rare medical condition called “menopause” had left her infertile, and she and Ben wanted him to help them bear a child; but instead of mixing their genes together, he took the opportunity to conduct his own experiment and create Sweetness, a true clone of Mel. When Mel found out the truth, she was furious, and in the heat of the argument Wakeling accidentally killed her. He tried to turn himself in at first, but when Sheriff refused to arrest him, he accepted what he’d done and has since come to see it as just another obstacle in the road to success. He came to Heritage when the jealous sniping of his colleagues got to be too much, and now he’ll simply pack up and move on again, to a world which can appreciate his genius.

Wakeling orders Bernard to kill the Doctor, but the Fussies hovering unnoticed in the corners of the room blow out blinding clouds of dust from their exhausts, enabling the Doctor to flee. Outside, Cole strikes down Ed with his cybernetic arm, and he and the Doctor flee together. Ace has taken Sweetness to the remains of the Heyworths’ farm, and the Doctor and Cole set off to join her, but they are pursued by the vengeful Wakeling and his thugs. The Doctor tries to lure them away while Cole takes Ace and Sweetness to his bar, but Bernard, Ed and Christa see Cole break away and follow, out for revenge. Far beneath them, Lee activates his last stick of dynamite... but at the last moment, he turns on the timer and runs, having decided to live with his crimes rather than taking the easy way out.

The Doctor confronts Wakeling in the ruins of the Heyworths’ farm. Wakeling isn’t proud of what he’s done, but insists that he’s doing this for the sake of humanity. However, the Doctor reveals that he’s not the first to create a true clone. The first clones created by humanity were used as disposable stunt doubles on movie sets. Throughout history the discovery has been made and forgotten, as those who learned to create life artificially forgot how precious it was. By causing the Heyworths’ deaths, Wakeling has forgotten what he was really working for, and this is how history will judge them. Furious, Wakeling attacks the Doctor, but as they struggle, Lee’s dynamite goes off, and the mines begin to collapse.

As Ace and Cole fight Bernard, Ed and Christa, Ed’s skittish horse steps onto unstable ground which gives way beneath it. Christa tries to save Ed from falling, but when Ace tries to help, Bernard drives her back, caring more about killing Ace than about saving his “friends”. When the dynamite goes off, Ed and Christa fall to their deaths. Ace manages to overpower Bernard and pull free the hose that connects his water reservoir to his walker; without this, he will dry out and die, and he’s forced to lie still, conserving his energy while Cole watches over him. At the farm, Wakeling also falls into the subsidence, and although the Doctor tries to rescue him, Sweetness walks up, clutching the murder weapon which Sheriff had thrown away. She drops it at Wakeling, who moves to grab it, perhaps instinctively or perhaps deliberately, and thus loses his grip and falls to his death.