A Scandroid robot crosses a huge junkyard and reports to field researcher Federer that 233 new files have been located from among the discarded computer technology that litters the area. Before he can link up to see them, his colleague Lake calls him on the communicator to see how he's getting on. Unfortunately he's found nothing useful so far, only books, chatroom archives and family pictures. He starts to download the new files and cries out in pain. He tells Lake that he keeps getting a stinging sensation every time a Scandroid links to him, so she recommends he move his dataport from his wrist as it's too close to the nerves. He agrees to book himself in with a cybertech when he gets back to the ship, but Lake says Dr Marriott wants him to stay out there a bit longer as he's convinced there's something of interest in that sector. Just then, another Scandroid arrives and announces it has priority information. Federer tells Lake he'll call her back and prepares to receive the data, but when the Scandroid connects to him he immediately screams in agony. He begs the robot to stop, but it's too late. As Federer falls to the ground dead, the Scandroid concludes that the information wasn't delivered and moves off to find a new recipient...

Not far away, two data pirates are sorting through some pickings aboard their ship. Gabe Stillinger tells his mother Denise that he found an I.D. backup chip sewn into the lining of a portmanteau case, but when she examines it, she tells him it's a fake. He knew that, but it looked pretty good so he thought they might be able to sell it on. She refuses to compromise her principles by selling fakes, even anonymously, but he reminds her that they need to sell something soon. He suggests moving to another sector, but she's convinced the clinic won't stay here much longer so they should ride it out. Before he can argue further, they hear the sound of the TARDIS materialising not far away. They look out through the door of their ship and see the blue police box on top of a mound of computer junk in this distance. The Doctor emerges and waves to them cheerily -- but as he introduces himself, his foot slips on something loose and he tumbles head first to the bottom of the pile. On the assumption that the stranger comes from the clinic, Denise suggests that Gabe bring him back to the ship while she contacts the other doctors to see if they can hold him to ransom and get some cash out of them. She may have principles, but that doesn't mean she has to keep to them!

Aboard the Lonway Clinic ship, Lake tries to contact Federer again, but gets no response. He can't have gone out of range in the twenty minutes since she last spoke to him, and Dr Marriott concludes that he's just slacking and will no doubt be back soon, claiming his com went dead. Lake isn't convinced as she heard him say that one of the Scandroids had found something. She's worried that the data pirates might have kidnapped him, but Dr Marriott doesn't care what she thinks and is convinced he knew Federer was a slacker from the moment he arrived. Unfortunately, their new accountant Tevez is arriving this afternoon -- the previous one, Nardini, disappeared with about six million from the company's profits -- and the last thing Marriott wants is Tevez thinking they're wasting time or money. Just then, Denise Stillinger contacts them to say she's holding one of their people hostage. She attaches a map directing them to a derelict shack about two klicks away, but warns them there'll be a price if they want him back. Dr Marriott knows Tevez won't be happy if they throw away good money getting back a replaceable resource like Federer, but he tells Claudia Bridge to take charge of the exchange. He grudgingly gives her a ransom budget of 5,000, but tells her that Federer isn't worth more than that, so if they won't accept the deal, she's to come straight back. Lake reports that Tevez's ship is coming in to land already, so Marriott goes to meet her and tells Bridge to avoid being spotted as she leaves.

Denise returns to Gabe, who is watching over their unconscious prisoner. They've tried to scan him, but the imprinter won't read him for some reason. The Doctor wakes up and demands to know what they're doing. He's twisted his ankle, but recognises their scanner as one that's used to take an imprint of the human brain. Denise assumes he's got an application in his brain to block the device and she offers to let him live if he agrees to hand it over. The Doctor refuses to respond to their threats and assures them his brain is just different. Denise's son has a cross-wired brain too, only in his case it's left him analogic, which means he can't process digital information. The Doctor realises he must be in the 32nd century as she's referring to organic digital transfer, or direct digital input into the brain. Gabe is beginning to suspect their prisoner doesn't belong to the clinic after all, but Denise points out that as the clinic has already agreed to meet them, they must have someone missing. In any case, if the Doctor is a moron they might want to conduct experiments on him. The two data pirates remove some gadgets from the Doctor's pockets to sell later. Denise tells Gabe to tie the Doctor up as it's time to set off for the rendezvous.

Dr Marriott greets Ms Tevez, who asks him to fill her in on what they do here; she's never had reason to visit the area before, nor did she have time to research it. He tells her that the Valley stretches for twelve klicks, and it's believed there may be four billion data storage devices currently here, with another sixty thousand items being dumped daily. The material ranges from the kind of chip you find in a refrigerator through to corporate hub systems. About 80% of it is either broken or stripped, but that still leaves a massive amount of recoverable data. Legally the policing of the area is a nightmare, which is ideal for their purposes. It was supposed to be part of a recycling programme, but it fell through decades ago and the authorities try to keep quiet about it. On the way in, Tevez noticed several floating homes and he tells her they belong to data pirates who hold private sector recycling licences. Most of them trade junk for appearance's sake, but they're really after the data as well... which makes working here a little dangerous.

Bridge arrives at the shack and calls out to the data pirates. Denise emerges from the shadows and demands to talk about money, but Bridge wants to see her colleague first. Gabe struggles out with the Doctor, who drops to the floor, and Bridge is understandably confused. Denise asks for 8,000 in return for the Doctor, and when Bridge claims that he's not one of their team, Denise assumes she's bluffing and threatens to kill him. Bridge genuinely doesn't seem to care and rebukes the pirates for wasting her time, then leaves. Denise fires a shot into the floor in an attempt to call Bridge's bluff, but when she doesn't return they realise she was telling the truth. No longer caring about the Doctor, the Stillingers take his possessions and leave as well, leaving the Doctor to his own devices.

Not far away, Bridge reports back to the ship and tells them about the hostage situation, dismissing the Doctor as a wandering nutter. She decides to stay out for a while to see if she can find out what happened to Federer, but as she signs off, the Doctor emerges from the shack and joins her. She notes that the pirates didn't kill him after all and he's surprised she's bothered enough to ask. Bridge explains that she'd never have been able to clear enough funds to bring back a non-staff member, so if the Stillingers followed through their threat she'd have had no choice but to stand by and watch them kill him. The Doctor is far from impressed. He asks where he is and she tells him this is Obsoletion Valley, the place where old computers come to die, something he regards as terribly wasteful. When he goes to show her the TARDIS, he discovers it has gone, and she tells him the pirates will steal anything. He offers to help her track down her missing colleague, but Bridge suspects he only wants to tag along so he can find his ship. Nevertheless, she agrees to let him come along for the ride so long as he doesn't annoy her. They both climb aboard her transport, a kind of hover-platform called a glider, and lift up into the air.

Elsewhere in Obsoletion Valley, one of the Scandroids tries to pass on priority information to the last member from another group of data pirates, but the man cries out in agony and falls dead to the floor alongside his colleagues. A second Scandroid arrives and asks if it needs any assistance, but the first robot only wishes to share its data. The two Scandroids connect, and then the new arrival announces it will return to the clinic to deliver the new priority information to its operators. The two robots split up and head off in different directions...

As the fly across the Valley, the Doctor tells Bridge how impressed he is by the glider, but she complains that the wind-resistance filter doesn't seem to be working. He remembers her saying she came from a clinic and he assumes she's a medic, but she tells him she's a researcher and a Scandroid technician. The clinic specialises in personality surgery and they're here looking for materials. The Doctor doesn't approve of artificially altering people's personalities to order, but he's particularly concerned that the clinic is using real people's personal details. Bridge says people often tend to be very careless when it comes to leaving backup brain prints on their old computer systems when they upgrade. The Doctor considers this to be theft, and although she says most of their clients only want to change a single aspect of themselves, the clinic does sometimes give clients someone else's full personality. The Doctor angrily claims that this is illegal and immoral, but Bridge assures him they always stay on the right side of the law. They also use a lot of redundant systems to provide inspiration for their aesthetic work and a lot of clients come to them looking for a more retro aspect to their personality. The Doctor had forgotten what a decadent age this is, but Bridge thinks that everyone who can afford it has had some work done. Even she's had alterations to keep her conscience in check and stop her regretting things. The Doctor suddenly spots the TARDIS below them, so Bridge agrees to take them down.

Tevez is impressed with the amount of work Dr Marriott's team have been doing, but unfortunately it's not enough to balance the cost, especially as the risk factor has pushed up the budget too much. She believes there are more efficient ways of achieving the same results and the operation is barely within the profit margins. Dr Marriott believes she hasn't taken into account what they might achieve in the future and he reveals that she doesn't know the real reason why he came here. He won't tell her what that reason is and asks her to take it on trust, but she knows that won't look very good on her report. He's a talented surgeon and people are already asking why he's spending so little time operating these days. He agrees to tell her the truth and she's surprised to hear him mention the name Zachary Kindell. Tevez dismisses Kindell as a volatile, immoral egotist, and although Marriott doesn't dispute this, he believes the world is now ready to learn from his research. Marriott is confident he'll be able to find some back-up copies of Kindell's mind in the Valley, and has been trying to trace his material for years. It's remotely possible they might even be able to bring Kindell himself back, but even if they can't, many of his projects may still be viable.

The glider comes down to ground level and the Doctor and Bridge discover a number of dead data pirates around the TARDIS. Clearly they were trying to steal the ship, and the wounds on the bodies suggest they were badly beaten, but if they were attacked by other raiders, why didn't they take any of their possessions? The Doctor produces an antique telescope and soon spots something about half a kilometre away. Bridge wants to take the glider, but he insists they get some exercise. They soon come across the dead body of her colleague Federer, but there's not a mark on him and the Doctor can't tell what killed him. Bridge suggests getting the body back to the ship so that one of the medics there can take a look, but then she realises that Federer's Scandroid isn't here. The robots are programmed to dig through all the junk and scan the storage units for material, but if a Scandroid's operator dies it's supposed to contact the ship and then wait with the body. So what's happened to this one...?

One of the Scandroids encounters a data pirate, but it dismisses him as a criminal and orders him not to interact with it or it will use violence against him. The pirate stops the robot with a disabling device, then he begins to access its data. The Scandroid tells him to prepare to accept priority information and it connects with his dataport. The pirate cries out in agony and tries to disengage, but it's too late and he collapses, dead. The Scandroid realises the information wasn't delivered and moves off in search of another recipient.

Tevez accepts that Dr Marriott's proposal is more interesting than she first thought, but she's not sure yet whether to recommend it. Even if he does find the material, they have no idea whether it'll have any value, as Kindell died decades ago, so any breakthroughs he made will probably have been surpassed by now. Marriott doesn't believe that, and Tevez agrees that she's starting to look upon his project more favourably. Just then, Lake calls to say they've picked up Federer's Scandroid on its way back to the ship, but there's still no sign of Federer himself. Marriott tells her to download whatever information it's got, then he and Tevez head down to join them.

The Doctor and Bridge find the body of another dead data pirate, but once again there's no sign of what caused his death. Bridge can see that this one definitely came into contact with a Scandroid because he's holding a device that can disable them and it's been discharged recently. The effect doesn't last long and the clinic don't own any as they're illegal. The Doctor is surprised that this should stop them, but she tells him it's not worth the clinic drawing attention to itself with something small like this. The Doctor decides he wants to go back to their ship as he wants to examine the Scandroid for himself. As they take off again in the glider, Bridge registers the Doctor's ID so she can pass him off as temporary staff; otherwise, the security system would try to apprehend him as soon as they enter the ship. She warns him not to cause any trouble once they're inside as she'll be held responsible for his actions. She becomes concerned when the ship doesn't respond to her hails...

Lake arrives in the ship's hangar ready to rebuke the technician Sampson for starting work on the returning Scandroid before she'd got there -- but she's shocked to discover the Scandroid standing over his dead body. The robot strides towards her, determined to download its priority information, but she orders it to keep back. She flees down the corridor and bumps into Dr Marriott and Tevez who demand to know what's going on. Lake explains that Sampson died after the Scandroid linked with him, but they think that's impossible. Marriott orders her to lock down the whole ship so no one can enter or leave until the crisis is over. Lake realises they haven't heard from any of the researchers in the field for over an hour, except for Bridge, who's coming in to land now. But Bridge is heading towards the hangar with the Scandroid...

The Scandroid finds that the Doctor isn't equipped with the standard dataport, so instead it tells him to prepare for a direct short-range broadcast. The Doctor calls out to Bridge to use the disabling device, which she uses to stop the robot. As the Doctor recovers, he asks her to wipe the Scandroid's memory and reset the machine back to its original factory setting. Dr Marriott's voice comes over the intercom -- he's been watching events on the monitor and can't believe Bridge has agreed to the Doctor's request, but the Doctor points out that if they don't remove the data on the Scandroid soon, it's likely to kill them all. Marriott doesn't care what he thinks and insists the robot be thoroughly debriefed first in case it's carrying vital information. The Doctor wonders why he hasn't come in to do that personally, but Marriott says that's what he pays other people to do. The memory wipe is completed and the Scandroid returns to its standard software. Marriott lifts the lockdown and warns Bridge that she hasn't heard the last of this.

Another Scandroid searches for someone to download its priority information to. Gabe and Denise Stillinger watch from behind cover and prepare to take a shot at it. Gabe chooses his moment carefully, then opens fire, but it has little effect so he fires again while Denise moves in closer with the disabling device. Eventually they manage to deactivate the machine...

Dr Marriott asks Lake whether she managed to get anything from the Scandroid before it attacked, but all she knows is that it claimed to be carrying priority information. Tevez wonders why it didn't simply broadcast the information back to the ship, but Marriott says anything important is always delivered in person as there's too much risk of the data being intercepted. Lake has traced the Scandroid's movements and discovered that it encountered two other robots on the way here and shared its data with them. Both of those robots went on to encounter others and now all those currently in the field have the same information -- and Lake can't raise any of their operators. Although the Scandroids aren't responding to remote control, most of them seem to be heading back to the ship anyway.

Denise tries to access the Scandroid's data, but it reactivates and orders her to stop. Gabe gives it another zap with the disabler, but he's worried it won't have much charge left. The Scandroid tells them to prepare to receive priority information, but as it starts downloading, Denise calls to her son to make it stop. Gabe watches in horror as his mother screams out and dies. The Scandroid then turns its attention to him, but when it grabs him it discovers he isn't equipped with the standard dataport. Instead, it broadcasts the information directly to him at short range, and although Gabe is analogic and can't interpret the data, the robot seems satisfied that it's been successfully delivered and steps back to await Gabe's instructions. Angrily, he orders it to shut up and the Scandroid accepts the order and deactivates. Gabe rushes over to his mother's body, but there's nothing he can do.

Tevez confronts Dr Marriott with the fact that he's not in control of things here, but he insists that the Scandroids are doing exactly what they've been instructed to do. Apart from the bit where they kill people, of course. As far as they know only one of them has done that, so he assumes it's probably just a mechanical fault. The Doctor and Bridge burst in and demand to know why Marriott is taking the ship off lockdown, and he tells them the other Scandroids are returning with vital information. He says he programmed the robots to behave this way if they found something relating to the clinic's key objective. The Doctor thinks he's insane as it's obvious the Scandroids have been compromised. Four people died in the hangar, so something outside has turned the robots into killers! Marriott demands to know who the Doctor is and what he's doing here, but Tevez agrees with the Doctor when he says they can't allow the Scandroids back inside. The fact that they can't contact their researchers in the field suggests they've all been killed. She's seen enough and orders Marriott to abandon the project, if for no other reason that the human resources cost is unacceptably high. Marriott refuses, but she warns him he'll never work in this industry again unless he listens to her. He insists they're throwing away the biggest advance in history and storms out. The Doctor asks Tevez if she has the authority to relieve him of command, but she's only an accountant and the best she can do is cut off his funding. Officially this is Marriott's project and he has control of the whole ship. Tevez plans to get in touch with the main office to get clearance from their superior, so the Doctor decides to try to reason with Marriott.

There are now nine Scandroids outside the ship waiting to get in, so Lake allows one of them admittance and orders the others to stay where they are. Unfortunately, when the hangar door is opened, all nine storm in and refuse to accept her instructions. The robots start making connections and one of the technicians dies instantly. Lake orders everyone to evacuate and then closes and locks the inner door behind them. Nearby, the Doctor catches up with Marriott and they both exchange warnings. Suddenly Lake and the surviving technicians race towards them and she tells them what's happened. Unfortunately the Scandroids were able to open the door because technically they outrank Lake. The Doctor urges Marriott to use his senior authority to close the doors again, but while he's making his mind up, the robots appear in the corridor and charge towards them. The Doctor still has the disabler in his pocket and he manages to stall the lead robot. While the corridor is blocked, Marriott finally agrees to move everyone back behind the next door, which he then seals shut.

Tevez has served notice of the problem to the main office, but she hasn't been able to get hold of anyone who can give her clearance. The Doctor joins them and asks if it's possible that the Scandroids are using electricity to kill people, but it appears they'd have to be substantially modified in order to do that. He wonders whether they've been deliberately programmed to do this in order to protect the data, but Marriott assures them he didn't do it. They should be relatively safe here, but the Scandroids are now between them and the exits and they've also lost their only disabler, as the man using it was killed. The controls that would switch the robots off have been overridden, something Marriott did earlier to make sure they could deliver their information. He also programmed them to deliver their data to the nearest authorised person, whereupon it would unlock an elite access code, and they're under orders to copy the data to any other Scandroids they encounter. The Doctor wonders if those present can de-authorise themselves so they can slip past the robots, but Bridge tells him the ship's security system will treat them as intruders and apprehend them if they do that. Also, if the Scandroids perceive a threat, they're programmed to resist using any means necessary, including violence where unauthorised people are involved. It's all starting to make sense to the Doctor. The rogue Scandroid beat up the first two data pirates that he and Bridge found, but the third pirate must have hacked it and received the data. This means the Scandroid hadn't been re-programmed as they originally thought -- it was actually trying to deliver the data in accordance with its instructions. It's the data itself that's the killer, not the robots! Unfortunately anyone who discovers what the data contains will be killed.

Gabe desperately tries to contact the clinic and eventually gets through to Lake. He tells her one of their robots just killed his mum so he's on his way to their ship now and wants to speak to the boss. Dr Marriott cuts into the communication and tells him they're killing his people too. Gabe reveals that his Scandroid has gone quiet now so they can have it back...for a price! The Doctor cuts in as well and asks how he managed to capture the robot, and Gabe explains that it tried to communicate with him and stopped when he wasn't able to understand it. The Doctor is delighted and switches off, leaving Gabe wondering whether he's going to get paid for being such a help.

The Doctor now has a plan. Gabe survived his encounter with the Scandroids, even though it killed his mother, which confirms the Doctor's theory -- it is the data that's killing people. Gabe suffers from analogia, a condition like dyslexia; while everyone else is able to connect computers directly into their brain, he still has to talk and type, and the Scandroids weren't set up to deal with such a situation. The Doctor rummages around until he finds some liquid hardware, which he then uses as the basis for a device to give themselves the same protection as Gabe. They need to physically stop themselves from understanding what the Scandroids are trying to tell them, and fortunately certain frequencies used in tandem can disrupt the areas of the brain that absorb this information. He need to test the theory on something that uses organic digital transfer, so Tevez produces a notebook and sets it to download. The Doctor activates the device and asks her to tell him as soon as the data becomes unreadable. It works almost straight away, so the Doctor prepares to test it on the real thing. Bridge is worried he might skip out as soon as he passes the Scandroids, but he assures her he's not that kind of fellow.

Marriott contacts Lake and authorises the door to be opened, allowing the Doctor to pass through. The Doctor is now alone with the Scandroid on the other side. It immediately recognises that he doesn't have the standard dataport, and begins broadcasting its information directly into his brain. Moments later, it concludes that the download was successful and stands down to await further instructions. A second Scandroid appears and begins the entire process again, and the Doctor realises his plan could take a while. He communicates with Tevez and explains what's happening, adding that he's wiping the Scandroids' memories as he goes. She congratulates him and tells him he'll be well rewarded, which infuriates Marriott. She then tells Marriott to start packing. There are four or five Scandroids still left, but Lake notices there are even more in the reception area. The Doctor agrees to tackle them next.

Alone in his office, Dr Marriott contacts Gabe, who has entered a holding pattern as he doesn't want to get too close to the clinic until he knows it's safe. Marriott promises to give him clearance to land and agrees to negotiate a price with Gabe later. He tells him it's the data he's after, so he asks Gabe to download it onto an old interface computer and bring it directly to him.

The last of the Scandroids aboard the clinic shuts down after downloading its information, but there are still more outside the ship. Before the Doctor can wipe the last one's memory, Bridge suggests they examine it first in case they can work out a way to stop this happening again. Reluctantly the Doctor agrees, but insists that they don't show the results to anyone else and they use a computer that doesn't operate via organic digital transfer. Unfortunately they don't have anything like that aboard the ship, so he decides it's too dangerous. Just then, the hangar door opens and Gabe enters. He tells them he's returning the Scandroid which is outside in his home, but he won't let Bridge have access to it until they agree a price. The Doctor is amazed that the Scandroid has killed Gabe's mother, yet his first thought is how much can he sell it for. Bridge recalls that Gabe uses old-style computers and asks if he's willing to rent one out to them.

Later, Gabe arrives outside Dr Marriott's office and knocks on the door. He explains that because of his analogia, he can't use any of the equipment on the ship -- even the buzzer. Marriott lets him in and asks whether anyone saw him come aboard, but Gabe tells him it was only a couple of people and they were busy with the clearing up. Gabe starts to negotiate and is surprised when Marriott accepts the first price he offers. They make the exchange and Gabe hands him an old-fashioned touch screen. Marriott casually asks why Gabe doesn't get his condition treated, but he says he's never been able to afford to until now. Marriott starts to examine the information with interest...

Tevez still hasn't been able to get in touch with head office, but when she does she plans to ask them to hand over command of this ship to Bridge as she's been impressed by her level-headedness. In the meantime, Tevez goes back to her shuttle to pack. The Doctor has been examining the priority information from the Scandroid and he suddenly calls Bridge over. He shows her the personality recode data, which allows a programme to run itself without supervision and rewrites the receiver's brain as soon as it's been downloaded. Bridge thinks this is incredibly dangerous as you need to know the pattern of the brain before you start moving things around, but according to the Doctor this particular programme targets the autonomous brain, the part that controls breathing and so on, and wipes it clean! Bridge has never heard of anything that would make you forget how to breathe and nor has the Doctor, so he takes the liberty of deleting it completely. But then he finds something else...

Marriott is pleased by what he's found. Tevez didn't think Kindell had achieved anything new, but this is revolutionary stuff about auto-surgery. Gabe reminds him that it killed people, but Marriott thinks it was designed to do that. There are also conventional applications for the same technology contained within a main file hidden underneath the deadly one. The way they're linked makes it impossible to open the main one without first opening the one that kills you, so unless the receiver knew what to expect and opened it up on an old-fashioned system like Gabe's, they would die. Marriott had programmed the Scandroids to deliver the data at all costs, so that's what they've been doing! The man who designed this is long dead, but he left behind copies of his research and didn't want anyone to get their hands on it, so he set a booby trap. Gabe wonders why he would do this, and Marriott theorises that it would be useful for Kindell if he ever came back.

Aboard her shuttle, Tevez contacts Lake and asks how long it'll be before she can get clearance for take-off, but unfortunately the ship has a damaged stabiliser and they need to run a check which may take twenty minutes, so Tevez decides to return to the main ship to wait. As she opens the airlock door to leave, she's confronted by a Scandroid who steps forward to download priority information. Tevez calls Lake for help, but it's too late and the robot successfully delivers its data, then stands down to await further instructions. Lake asks Tevez if she's alright, and Tevez responds, but she seems momentarily confused. She switches off the communicator and asks the Scandroid to make a copy of the programme it delivered, then she needs it to tell her all about where she is...

Marriott deletes the dangerous file and imports the main data, then he hands the touch-screen back to Gabe. Marriott takes in the information and at first he's delighted -- but then he suddenly starts to act strangely and orders Gabe to leave. His voice changes and then his body begins to physically alter ...

Aboard the shuttle, the Scandroid is telling Tevez the history and purpose of the Lonway Clinic when it suddenly stops its lecture and declares that all scum have tiny minds and are unworthy. It lashes out at Tevez, clearly intending to destroy her, and refuses to obey her instructions to stop. She races from her shuttle, with the Scandroid in hot pursuit...

The Doctor, Bridge and Gabe race down the corridor, hoping Marriott didn't have time to lock the doors down. They bump into Tevez and warn her not to go forward as something strange has happened to Marriott, but she in turn warns them not to go back as something strange has happened to the Scandroid behind her. They see the robot approaching and the Doctor realises Marriott has turned the Scandroids against them. As they race for the hangar, the Doctor explains that the programme Marriott accessed not only changed his mind, it also reshaped his body. The result they saw in his office was unfinished, which suggests there must have been a mutation in the programme that its creator couldn't solve. Offended, Tevez tells the Doctor that she was very close to resolving it, but before he can question her further, they arrive in the hangar and find Lake waiting for them. She tries to warn them about the Scandroids, but suddenly one of them attacks her. The Doctor and Gabe race forward to pull her free and then they flee to an area of the ship that contains no robots. The Doctor takes Gabe's computer and patches it into the ship's systems so he can lock the doors. Bridge doesn't think this will work as Marriott still has control of the doors, but the Doctor is confident he can be more persuasive. Gabe wonders why the ship still recognises Marriott in his mutated form and the Doctor explains that as far as the ship is concerned, living things are always changing. The computer would have witnessed his change and adjusted its view accordingly. That's one of the problems with artificial intelligences -- they're far too smart, just when you don't want them to be. The Doctor finishes re-programming the doors, but the others are worried they may not get them open again as they're now trapped inside a tiny room. He asks about Lake, but sadly it was too late for her and she's dead. The Doctor turns his attention back to Tevez and remarks on how different she seems, although he can't quite put his finger on how. The woman confirms that her name isn't Tevez -- it's Dr Zachary Kindell...

The Scandroids approach their new master, the mutated Dr Marriott, who asks where the scum are. The robots direct him to the locked room, but when Marriott orders the door to open, it remains resolutely shut. Inside the room, the trapped survivors listen as the robots throw themselves against it. Kindell explains that the mutant is the best result he's had, but the Doctor is more interested in the auto-surgery programme that enabled Dr Kindell to hijack Tevez's body. Kindell explains that one of the Scandroids found his programme out in the Valley and delivered it to the nearest functioning human. The programme was designed to erase the brain pattern of the recipient and replace it with Kindell's. All the other Scandroids carried the killer programme but Kindell made the two programmes mutually exclusive so that his research would remain safe while still allowing the personality print to find a new body. The Scandroid that found Kindell's personality didn't hold a copy of the booby-trapped research, and it was further afield than the others and therefore took longer to return to the ship. Gabe wonders why Kindell left the material here of all places, but he says he just left copies scattered around and hadn't expected it would take so long to get found.

The Doctor is aware of Zachary Kindell's work; he was a pioneer of personality surgery, but his reputation was tarnished by a lack of responsibility. Tevez's body now has Kindell's entire personality inside her, including all his memories. The Marriott-mutant creature outside was the result of another programme that was designed to rewrite the host's DNA and instantaneously transform their body, but unfortunately it only hones in on certain parts of the brain, which causes an imbalance. Kindell thinks he could have fixed the problem if only he hadn't constantly had to work under threat of prosecution. He becomes angry at what he feels was unjust treatment; he had many supporters and believes that one day the deaths he caused will be just a footnote in history. The Doctor is furious with his dismissive attitude. Kindell argues that the programme simply makes people's outward appearances reflect their true selves, but the Doctor believes it actually unleashed people's aggression, hate and ugliness, and allowed that to transform them instead. Kindell says the mutants are unfocussed rage and are extremely hard to kill. The volunteers he previously used as guinea-pigs all knew the risks and were equipped with a suicide implant, but there's no way of reversing the process. Kindell recommends destroying the Marriott-mutant.

Marriott is determined to open the locked door, but there appears to be no way to do that from the outside. Suddenly, the door opens of its own accord and Gabe stands there taunting Marriott and the Scandroids. In the momentary distraction, Bridge opens fire, but it seems to have no effect. The survivors hurriedly close the door again, and realise they've had a close call. Their weapon was useless and they don't have access to anything more powerful because the ship is only equipped to deal with disgruntled data pirates. There's only one mutant out there, so Gabe wonders if they can give it the run-around long enough to get away. They've already tried to override the Scandroid's programming and couldn't because of Marriott's special protocols, but now the robots are running on standard software, they should be able to give them commands. Kindell says it's unlikely the Marriott-mutant will have locked out their access as his earlier work suggested the mutants only react to situations rather than anticipate things. In short, Marriott won't think of locking them out until after they start issuing commands. Whatever they decide to do, it'll have to be something that will work before Marriott notices what they're doing. Erasing the Scandroids' memories would take too long, but the Doctor recalls that Kindell created a backup of the auto-surgery programme in case anything went wrong, so he suggests modifying it to rewrite a Scandroid's brain. Kindell thinks this should work, which means they can turn one of the Scandroids outside into another copy of himself. Bridge thinks she can get near enough to one of the Scandroid to use its access port, then hopefully it will deal with the others itself.

They open the door again and two Scandroids charge towards them. The Doctor orders Gabe to shoot one and let the second get inside. They immediately shut the door behind it and leap onto the robot, allowing Bridge to open its access port and upload Kindell's personality. Eventually the Scandroid strops struggling and asks them who they are. Kindell introduces himself, but the Scandroid disagrees and says that it is Kindell. The Kindell inside Tevez points out that the other one is just a robot with his 'brain', and when the truth slowly dawns on the new Kindell, he demands that he be transferred into a human brain immediately. The Tevez version orders the robot to help him, and when it refuses, he threatens to have it wiped.

Moments later, the door opens again and the Scandroid emerges. It approaches another robot and announces that it has information to pass on. They make contact and share the data, and then the second Scandroid becomes a third version of Kindell. Inside the room, the survivors are struggling to hear what's going on outside. They think their plan is working, but it's hard to say what's going to happen next. Robots can't deal with the way that human personalities fluctuate, so as the Kindell mind spreads amongst them there may be dissidents, but at least it will take them away from Marriott's control. The Doctor thinks the robots should all be infected within fifteen minutes, which upsets Kindell as he doesn't appreciate his personality being likened to a disease.

Before long the ship is filled with multiple robotic versions of Kindell, many of whom are confused. They see even more Scandroids and move forward to share their new information, and then the Marriott-mutant arrives and tries to take command. When the Doctor and the others emerge from the locked room, they're relieved to see the corridor is now clear. Keeping an eye out for any other survivors, they argue over their next move. Bridge recommends using the gliders from the hangar because they're fast, but Kindell thinks Tevez's ship should be safer and it's parked on the other side. The Doctor suggests they work out a way of stopping the Marriott-mutant first, but Bridge is more concerned with escaping as quickly as she can. Gabe rushes back after spotting Marriott fighting with the Scandroids, but then Marriott sees them too and charges towards them. They have no choice but to flee towards Gabe's home...

The group race inside and Gabe prepares for take-off. Bridge reminds them that Tevez's ship is still nearby and they might need it, so Gabe uses his retrieval net to drag it alongside his. Bridge still can't understand why they don't just leave, but the Doctor points out that nobody else knows how to stop the Marriott-mutant. Kindell agrees as his opportunities to study the mutants in the past were severely limited by the fact that they kept trying to kill him or each other. Before they can debate further, they hear Marriott arrive outside, so Gabe lifts off. Once they're safely in the air, Gabe admits that this is all his fault as he was the one who sold the computer to Marriott. Just then, the home starts listing to one side and an alarm sounds. To their horror, they realise the mutant must have grabbed onto the side of the ship as they launched. They can't seem to shake it off, but Gabe thinks they can land and escape in Tevez's ship. The Doctor tells them he has another idea, although it's something he really didn't want to do. While he works things out, Gabe lands his home -- then takes out his computer and, without warning, plugs it straight into 'Tevez'. Kindell designed the programme in the first place, so Gabe thinks it's time the programme had a go at re-designing him. The Doctor protests, but it's too late -- and within seconds, the body of Tevez, containing the mind of Kindell, starts to physically mutate.

Gabe and Bridge race off the flight deck, pausing only to encourage the Doctor to follow them. Gabe leads them to his mother's room which contains an emergency escape hatch. Tevez's shuttle is locked onto it, so the three of them climb through and get aboard it. Bridge thinks she should be able to fly Tevez's shuttle, but she'll have to hack into it first. The Doctor is furious with Gabe, but the young pirate claims he did it to give the Doctor the time he needed. In any case, Kindell told them earlier that the mutants always seem to hate each other, so with any luck the two here will both destroy themselves. The Doctor is doubtful -- he makes it a general rule not to create one monster to destroy another and he certainly wouldn't sacrifice a human life to do it. Gabe insists that he made the right choice, although he did consider for a moment that he should sacrifice Bridge instead, given the fact that she keeps moaning all the time and she doesn't have a sense of responsibility. The two start arguing, but the Doctor shuts them up. For better or worse, he now has the breathing space he needed. He asks Gabe to hand over his computer...

The mutant Kindell-Tevez trawls through the corridors of the Lonway ship searching for scum and before long it encounters the Marriott-mutant doing the same thing. They start to fight each other...

Bridge asks the Doctor what he's doing and he tells her he's finishing Kindell's project. Gabe is amazed that he'd want to do such a thing, while Bridge is amazed that he thinks he can improve on the work that one of the field's greatest scientists died trying to resolve. The Doctor thinks that Kindell may have been too close to the problem and sometimes what you need is the eye of a talented amateur. Gabe patches into the clinic's security system and they watch on the monitor as the two mutants knock the stuffing out of each other. The creatures don't seem to be tiring and Gabe considers having a wager on the winner, but it's impossible to tell them apart. Eventually the Doctor finishes his work, but unfortunately he has no way of testing it. Bridge wonders why the Doctor isn't insanely rich if he can do this sort of thing, but he tells her there are lots of things he can do that he chooses not to. He's going to need their help, but although Gabe steps forward voluntarily, Bridge refuses to be co-opted into anything. The Doctor realises her lack of conscience means she's free to act in any way she likes without feeling guilty later, and Gabe realises for the first time that she's had her brain modified. The Doctor wonders if she realises there's something missing in her life, and if she ever remembers how good it feels to do what's right? Bridge deduces that if she can help get Tevez back, the boss might treat her more leniently when they return home, so she agrees to help the Doctor, albeit not for the right reasons. The Doctor tells them that Marriott and Tevez were both equipped with dataports, so if they can get close enough they can feed the new programme in. This will rewrite their brains and make their bodies follow suit, as Kindell originally intended -- only in this case they'll become human again. But first, they need to hold the mutants off for long enough to insert the programme. Gabe says their hand laser still has some charge left and he has a couple of other weapons with stun settings. They'll just have to hope that's enough...

The Doctor, Gabe and Bridge emerge into the corridor where the two mutants are fighting. Gabe shoots one of the creatures and it falls to the ground, then he tries his best to hold the other one off long enough to give the Doctor and Bridge time. Bridge struggles to find the mutant's dataport, as everyone has them fitted in different places, and the mutation has made things even more confusing. The Doctor takes the stun weapon and uses it to stop the creature struggling, but Gabe isn't finding it easy to keep the other one at bay. Eventually Bridge finds the dataport and the Doctor attaches the computer. Before the mutant can recover, they turn their attention to the second one and after a short fight, they manage to subdue it long enough to start downloading the programme again. The three of them turn to flee back to the shuttle while the programme gets to work, but on the way the computer is smashed and Gabe is knocked unconscious.

The Doctor and Bridge help carry Gabe back, but in the confusion they didn't get a chance to see whether the programme was having any effect. From outward appearances Gabe doesn't seem to have suffered any permanent damage, but you can never tell with blows to the brain. Once again, Bridge tells the Doctor she can't believe that someone with his talent chooses not to use it, but he tells her he's seen technology go wrong too many times. He just hopes he hasn't made a mistake finishing Kindell's work. They realise the noise coming from the corridor has stopped and Bridge is confident their plan has worked. They go outside and find the mutants have returned to a human state, but they now both inhabit identical copies of Tevez's body. The Doctor and Bridge manage to separate the two 'women' who are continuing to fight despite the changes they've undergone. The Doctor explains that he used a backup copy of Tevez's brain-print from her shuttle, which is why both mutants now look like her, but he hopes to change Marriott back to his original form later. Unfortunately it might be a bit late for that now as one of the two Tevezes is badly beaten, and moments later she dies. The surviving Tevez recovers enough to see a second version of herself lying dead on the floor, and starts to wonder who she really is...?

Gabe starts to recover and he asks about Marriott, but his question is answered by Tevez, who reveals that the Doctor could only save one of them. The Doctor has deleted every copy of the project Marriott was looking for, and both Tevez and Bridge approve. Bridge decides to stay here for a while looking for any copies that may have been overlooked and although the Doctor is sceptical, she assures him it's just so she can get rid of it and make sure no one else finds it. He finds her attitude almost conscientious and teases her about getting it seen to before she starts caring about things left, right and centre. Gabe agrees to let her stay in his home for a bit, but adds that he's quitting the business soon as he's not really cut out for it. The Doctor advises him to get some rest and asks Bridge to look after him.

1974: A young woman, Lauren Hudson, returns home and makes a call to her mum. She dials the number, but the voice on the other end is that of a stranger. She apologises and says she must have rung the wrong number, which is a coincidence as she received a wrong call herself just half an hour earlier. The man says he just happened to be walking past a telephone box when he heard it ring. There was no one else around and he was intrigued, so he answered it. When he introduces himself as the Doctor, Lauren wonders whether he can help her. She's feeling rather unwell and was trying to ring her mother for advice. Despite the Doctor's reluctance, she insists on listing her symptoms -- and he starts to take more interest when she tells him the skin around her fingernails is broken and bleeding. As she tells him more, his voice becomes urgent and he tells to hurry to a hospital. They'll probably just tell her it's food poisoning, so he pleads with her not to leave until they've given her a thorough check-up. He advises her to insist they look particularly at her neck and if they refuse, she must kick up as much of a fuss as she has to. Although she's confused, Lauren agrees, so he wishes her good luck and tells her to leave immediately.

A couple of weeks later, Lauren makes another call and asks to speak to Mr Deacons. The man who answers the phone tells her she has the wrong number, but his voice seems rather familiar. It's the Doctor again and they're both amazed at the coincidence. He's glad to hear that she's alive and well and she thanks him for his earlier advice. She tells him she went straight to the hospital and kicked up a fuss, just as he suggested. As she's in her booth at work, where she's a telephone operator, she lowers her voice when she gets to a bit of the story that she's not supposed to talk about. She says the doctors looked at her neck and everyone went quiet, then eventually she was put in quarantine for a couple of hours until someone from the Army came in, accompanied by another doctor who she hadn't seen before. The Doctor wonders if he knows the man she's talking about and asks if it was a tall chap with white hair and flamboyant clothes, but she says it wasn't. The doctor at the hospital told her he was going to have to operate on her, and when she came round afterwards, the Army man made her sign the Official Secrets Act. The Doctor advises her not to say any more in case she gets into trouble, but she insists on continuing.

After the Army man left, the hospital doctor showed Lauren a little plastic tube containing a metal worm or snake, about six inches long and with legs all along it. She has no idea how it could have got inside her, but the doctor said it had been hugging onto her spine. If she hadn't had the operation, she'd have been dead inside a day. The Doctor is delighted she made it in time. After she went home, she felt really drained for a couple of days, but now she's fine and can hardly believe it even happened. She's curious as to how the Doctor diagnosed her problem just from what she told him over the phone, but he puts it down to experience. Lauren asks him if the worm was alien, and when he admits that it was, she realises this means he has experience of aliens. She's been wanting to thank him for weeks and she can't believe that she called him again by mistake. The Doctor suspects it may not have been a coincidence, but neither does he believe in fate. Lauren can hear someone enter the room where the Doctor is and he admits that he's actually waiting in someone else's office and he picked up the phone when it rang so he could take a message. She thinks picking up other people's phones seems to be a compulsion for the Doctor, but he assures her it hardly ever happens. Lauren gives him her name and asks how she can get in touch with him again, but he tells her he doesn't have a phone and has no fixed abode. He starts to make excuses to get her off the phone and after saying goodbye, he hangs up rather abruptly.

Some time later Lauren makes another phone call, this time to her friend Connie, but is amazed when she gets the Doctor again! He too is delighted to hear from her, despite the fact that the last time it sounded like he couldn't wait to get her off the phone. He apologises for his behaviour and says he was busy at the time, plus he can't keep in touch with everybody that he saves or he'd never get anything done. Lauren can't believe she's called him again and he agrees that it's odd, but it's definitely not a coincidence. After the last time they spoke, the Doctor started thinking about all the factors involved in the two previous calls, not least the fact that she twice got through to the one person in the whole world who might have known what was wrong with her. Lauren points out that the second call came when she was really keen to thank him, so on both occasions she had a strong reason for wanting to speak to him personally. Bearing in mind the fact that two different telephones were involved, neither of which belonged to the Doctor, he's convinced the coincidences were actually the result of a virus. After he spoke to her, he made other phone calls (which is rare in itself) and on both occasions he got wrong numbers too, but the people he spoke to just happened to be very useful leads in another 'strange matter' he's been looking into.

Lauren is still curious about the parasite in her neck, and the Doctor tells her that if she hadn't got it out in time, she'd have died and the parasite would have processed the raw material of her body. Within hours she would have turned into a slug creature. He got to the bottom of the matter and stopped them, but he's not sure he would have done if he hadn't had those wrong numbers. He decided to run some tests on himself and found traces of a very sophisticated bio-engineered virus that he thinks was transmitted by telephone. When someone catches it, it causes them to make subconscious mistakes the next time they use a telephone. They dial a wrong number, but the virus also generates coincidences, so the person they call is someone who can help them or who they can help. Then that person catches the virus and so on until it reaches the end of its lifespan. Lauren doesn't think this sounds even remotely possible, but the Doctor reminds her this is an engineered alien virus created by technology far in advance of anything she can imagine. It's incredibly versatile and has adapted to the way humans communicate. He doesn't know what it's doing on Earth -- it may have even ended up on here accidentally -- but he intends to find out. Lauren has been thinking a lot about aliens recently and now finds that most other problems in life aren't worth worrying about. The Doctor understands what she means and says she'll probably never see things the same way again, but he's sorry that she's finding things a bit boring now. He asks her if she'd like to help him find out where the virus came from. All she has to do is make a note of all the wrong numbers she gets from now on, then take down their names and numbers so he can trace them back to their source. She agrees to do her best and the Doctor says he's sure they'll talk again soon.

Sometime later, Lauren is working at the telephone exchange when a call comes through from the Doctor. Up until now, all the wrong numbers have been from Lauren to him, but now they've going the other way too, which suggests the virus is spreading. Lauren says also called a wrong number yesterday and it was the first time she didn't get put through to the Doctor. Instead, she ended up talking to a woman who tipped her off about a sale starting today and she got some good bargains. The Doctor asks for details of the last wrong number she called, and she tells him she spoke to Malcolm Taylor and was able to give him the title of a song he'd had stuck in his head for two days. He too had received some wrong numbers and was happy to help. In fact, he started coming up with elaborate theories about what was happening, all of which were wrong of course. The Doctor decides to call Malcolm himself, and Lauren is upset that he hasn't bothered to ask how she is. He's keen to get on with tracking down the virus, but she advises him that people might be more willing to help if he showed some social niceties. She tells him that she's been feeling dissatisfied with her job recently and wants a change of scene. She wants to be an actress, but she hasn't had a break so far and she's hoping the virus will bring her a bit of luck. The Doctor quotes a relevant passage from Thomas Jefferson and warns her she might have to wait a long time for her luck to change. He makes an excuse to hang up and she gets ready for the next wrong number...

The DJ of the County 6371 radio station tells his listeners they've received lots of calls for today's quiz. To make their first day on the air, they're offering a holiday for two in Marbella, with flights, hotel and meals all included, plus spending money. It's the biggest prize ever given away on British radio and it could go to their next caller if they get the answer to a question right. The DJ opens up the lines and the first person to come through is Lauren -- by mistake. Although she admits that she's come through to the wrong number, the DJ allows her to continue anyway. To win the prize, Lauren has to identify a quotation from a great American leader. Amazingly, it's exactly the same quote the Doctor used earlier and she can't help but burst out laughing...

Connie Pickerill answers her phone and finds the Doctor on the other end. When she introduces herself, he recognises her name and asks if he can speak to Lauren. Lauren comes on and the Doctor tells her he's being held at Green Street Police Station, although he's not particularly worried as this sort of thing happens all the time. It appears that he was arrested for trespassing, and she guesses that he turned up at some poor soul's doorstep, rambling about alien viruses. He's confident he can sort things out if only he can get in touch with a good solicitor, and that's what he was trying to do with his phone call. Unfortunately he doesn't have any money and needs to borrow some for a day. Lauren is delighted to help as she's just won a holiday thanks to him, so she agrees to arrange something in the morning. She even offers to go to the police station personally and bail him out, but he says a solicitor is all he needs. He then starts to make small talk and she congratulates him on improving his social skills. He thanks her for offering to help and then she returns to the party she's having with Connie to celebrate their forthcoming holiday.

When Lauren next answers the phone, she asks the Doctor if things are going well and he tells her the solicitor she arranged was excellent. She refuses to let him pay her back and says it can come from her part of the holiday spending money. His investigation is going well and he thinks he's close to finding the centre of all this activity. He asks again for the name and number of her last wrong number and she tells him it was someone called Julie Nicholls, who turned out to be an old school friend of hers that she'd lost touch with years ago. Lauren asks the Doctor whether they really want to get rid of the virus -- after all, it seems to be helping a lot of people and hasn't done anything bad. The Doctor thinks they should count themselves very lucky as the technology was undoubtedly intended for other purposes. It's a sad fact that all good technology is designed with military applications in mind and the virus was probably meant to be used by sleeper agents in enemy territory. It would enable them to speak to each other without having to carry around incriminating contact details and also randomly put them in touch with the people best placed to help them. He admits that his policy is never to let cultures keep hold of advanced technology that they're not capable of handling and Lauren can understand this. If the virus was adapted, it could be used to assassinate someone just by phoning them up. Even if the virus continues to be a positive thing for Lauren, he wonders if that's how she wants to live? He's glad it saved her life, but if everything is dropped in her lap without her having to work for it, it will start to feel unsettling and unsatisfying after a while. He tells her he always tries to do whatever he thinks is right, and one of those things is to make sure people don't get their hands on things they shouldn't. He sometimes has to make some big decisions, and Lauren doesn't think this can be easy. He decides to go and they say goodbye again.

Lauren makes another phone call -- choosing her friend Connie's number off the top of her head -- and is surprised when Connie actually answers. Lauren sounds disappointed and her friend realises something is wrong. Lauren explains that she just received a wrong number, so she thought she'd try to make a call too, but she was hoping to get someone else. Connie is understandably confused, but Lauren can't tell her the truth so she casually asks if their arrangements for Friday night are still on. They say goodbye and Lauren hangs up. Later she decides to write a letter instead: