A lone Weevil scurries through the night and makes its way into a warehouse. Moment later, Owen and Tosh arrive and follow the creature between the crates and glass dividers. The Weevil stops and looks curiously at something on the floor, but when its pursuers arrive it snarls and departs quickly. Owen and Tosh shine their torches down and see a man lying on a tarpaulin. Owen examines him and confirms that the man is dead.

Later, a visitor arrives at the tourist information booth at the front of Cardiff Bay. Ianto is at the reception desk, idling flicking through a magazine. He looks up at the visitor and tells them the office is closing, but the visitor produces an ID card and Ianto jumps to attention and apologises. He operates a control and the entire back wall of the office glides open to reveal a long, brick-walled corridor. He invites the visitor in…

In the medical area of the Hub, Jack and the others are examining the body of the man found in the warehouse. The man has been identified as Meredith Roberts, but because there are no obvious signs of violence it seems unlikely that the Weevil was responsible for his death. Ianto calls to say Jack’s VIP visitor has arrived, which surprises the others as they weren’t expecting anyone. They follow Jack into the main section and watch as the huge metal door slides open to reveal Ianto and a beautiful young woman. Jack introduces her as Miss Martha Jones and gives her a friendly wink.

Martha hugs Jack and tells him it’s good to see him again. Jack introduces her to the others, but when Owen asks if it’s just a casual visit she says she’s come to complete his post mortem. Jack explains that Dr Jones is from UNIT, which confuses Gwen as she keeps losing track of which group is which. Jack describes UNIT as military intelligence with cute red caps and says they’re the acceptable public face of intelligence gathering on aliens. He says Torchwood is more ad hoc, but he thinks they’re the better looking ones. Martha tells them UNIT has identified a pattern from their data on recent sudden deaths. Although all the victims died from toxic shock, there’s nothing else to link them and they’re all from different age groups, ethnic groups, occupations etc. However, there’s been a significant concentration on cases emerging from South Wales. So far they’ve been written off as suicides or accidents, but when she examines the dead body in the autopsy room she shows everyone a small puncture wound in the victim’s eye. All the other victims had the same wound - it’s from a hypodermic needle and each of the bodies’ blood streams had been pumped full of ammonium hydroxide. Owen says he was planning to check the blood stream next and when Martha asks if he’s checked the victim’s medical records he says he was just about to. Martha teases Owen and tells him to stick with her as he might learn something.

The group check the database of the Royal Cardiff Infirmary to look for Meredith Roberts’ file. Tosh says she knows there’s been a major crash on the NHS recently and lots of files have been lost, but Martha points out they include all the victims she flagged up from the UNIT data. Not surprisingly, Meredith Roberts’ file is also listed as irretrievable. No crash would delete data so cleanly, so it’s obviously deliberate and Tosh decides to look into it further.

Martha joins Jack in his office and he asks her how her family are. She says they’re getting better and they send their love. They’re all exclusive members of the ’end of the world survivors club’ and Martha tells Jack she’s glad to see him again. Then Jack asks if Martha misses “him“ but she says she doesn’t - well, maybe sometimes just a tiny bit, but then she always comes to her senses and realises she made the right choice. Now that she’s a medical officer for UNIT she has plenty to occupy her life and she teases Jack that he has to follow her orders to the letter. He tells her she should have called him if she was looking for a job, but she explains that a woman from UNIT called her out of the blue and said she was just what they needed and that she‘d come highly recommended by an impeccable source. They both know who they’re talking about and Jack realises “he” must have thought he owed Martha a favour. Jack says they all do. Then he asks Martha if she could get him one of those red caps, just for personal use as he thinks Ianto will look good in it. Martha changes the subject and asks for a guided tour.

Gwen accompanies Jack and Martha on the tour of the Hub and as soon as she gets a chance, she asks Martha how well she really knows Jack. Martha tells her they were only together for a few days, but it was pretty intense. She sees the look on Gwen’s face and realises she’s given the wrong impression about her relationship with Jack. She quickly clarifies and Gwen says the same, which Martha says must make them the only two people on the planet who aren’t. When Jack asks them what they’re talking about, they claim it was just a discussion about alien flora. Jack thinks they should raise their game.

Jack shows Martha some of the alien technology they’ve picked up over the years and Martha agrees they have some impressive kit. One particular item grabs her eye, but Owen says there’s been a lot of argument about what it actually does. Owen’s best guess is that it’s a surgical instrument and he’s decided to call it a Singularity Scalpel. He says it concentrates energy on a tiny fixed point without damaging anything in the way. He demonstrates by screwing up a piece of paper and placing it inside a paper cup. His plan is to vaporise the paper without scorching the cup. Gwen warns him about what happened the last time he tried this and even Jack thinks it’s all going to end in tears. Owen adjusts the alien device and activates it - and an object on the other side of the room explodes, causing Ianto to scream out as he approaches them. Everyone laughs and Owen admits he hasn’t got the calibration worked out yet. Ianto reports that there’s been another attack. A 27 year old woman has been assaulted with a hypodermic needle - but this time she survived and is in hospital. He’s already checked and her medical records have been wiped just like all the others.

At the hospital, Martha and the others conduct further tests on the bemused patient, Marie. She’d been assured by the doctors that they’d finished with her, but Martha tells her they’re a bit different. Gwen asks Marie if she knows who attacked her, but she says she’d never seen the man before. He didn’t say anything during the attack and he just came towards her with a needle, but her dog took a bite out of him and she kicked him in the nuts. Jack is seriously impressed.

Back at the Hub, Martha and Owen are working extremely well together as a team. They spend hours running a series of tests on the samples taken from Marie and while they’re waiting for the results to come through, Owen takes the opportunity to ask her more about Jack. He knows they go back a long way, but Martha says they go forward as well as back! Owen asks what brought them together and Martha says they were under the same doctor. She turns their attention back to the murders and wonders why the attacker is using ammonium hydroxide as it‘s a weird way to kill someone. It’s like injecting someone with bleach. They suddenly realise the objective isn’t just to kill the victim, it’s to destroy something in the victims’ blood stream - which means the weapon was chosen to get rid of evidence!

Jack and the others review what they know. They have a victim and a survivor, both local, both attacked with a hypodermic and both with their medical records wiped. There have been more cases throughout the UK and it looks like the attacks are being done to remove evidence of a particular medical condition. Jack asks Gwen and Ianto to pursue the criminal investigation while Martha and Owen follow up the medical side. He asks Tosh to continue her work in trying to retrieve the medical records, but she’s going to have to do a lot of illegal digging. Jack tells everyone to do whatever is necessary - they’re no longer dealing with just a random killer, this is part of a conspiracy.

Tosh learns that the police have discovered another body in Heath Park and from the initial reports it has all the hallmarks of the other attacks. Gwen and Ianto race to the scene which has been cordoned off by the police and is in the process of a forensic examination. Gwen checks with an officer who tells them the body of a man in his early 20s was found earlier this morning. He’s been identified as Barry Leonard, a student, and the doctors believe the cause of death was toxic shock.

Back at the Hub Gwen tells Martha the attack took place in the middle of the woods, so there were no witnesses and no CCTV footage to examine. As with the other victims, his medical records have been wiped and his body has an identical puncture mark on his eyeball. Martha thinks the attacks are more like professional assassinations, but who would want to assassinate a student? The victims must have something in common and the fact that their medical records have been wiped is obviously a clue, but until Owen finishes analysis the test results, they’re still in the dark. The attacks are getting more frequent and they have no idea who will be next. Just then, Ianto contacts Martha and says the hospital have called to say Marie has had some sort of seizure. Martha collects Owen and he tells her he’s found something unusual about Marie’s blood sample. Martha suggests running an iso-enzyme analysis as it could be a parasitic infection. They double check Marie’s data but find no infections, no deficiencies in her organ functions, perfect cholesterol levels and ideal blood pressure. In fact, there’s absolutely nothing wrong with her - she’s so normal, she’s abnormal…

Gwen and Ianto begin interviewing some of Barry Leonard’s student friends. One of them in particular, a student named Mike, knew Barry very well as they were on the same course together and became best mates. Gwen asks whether Barry had been taking drugs, but Mike says he always used to be really careful about what went into his body because of his diabetes, but after he was cured he started drinking and taking blow. Ianto tells Mike it’s not possible to cure diabetes, but he insists that Barry had stopped taking his insulin and was fine. Gwen asks who performed this miracle cure, but Mike says Barry was always very secretive about that, but around the same time he suddenly had lots of money to flash about.

At the hospital, Martha checks on Marie’s condition and Owen tells her she’s got something in her blood they don’t recognise and they think it might have something to do with her current illness. They say they need to know what’s been happening to her in order for them to help her. Owen takes a call from Ianto and learns that Barry Leonard had apparently recovered from his diabetes. Owen asks Marie if she’s ever had a serious medical condition and she admits that she used to have HIV. Martha says that’s not medically possible as her blood is completely clear of antibodies, but Marie says that’s a result of a drug called reset. Owen asks her for more information and she says she got it from a research facility called the Pharm. She says they gave her a lot of money but it was part of her contract to keep quiet about it. Marie starts coughing and then suddenly she goes into seizure and thrashes about on the bed violently. Martha and Owen try to help her, but within seconds Marie has died. They look at her in bafflement…and then they notice movement inside Marie’s mouth and thousands of tiny insects emerge and fill the room. Owen and Martha dive for cover into the corner of the room and place masks over their noses and mouths. The insects swarm around for a few seconds and then just as suddenly, they start dropping to the floor, dead. Owen picks up one of the insects and looks at it in amazement…

Analysis back at the Hub shows that they’re dealing with alien larvae that can incubate inside human bodies. Owen says they’ve never seen anything like it before. The insects left Marie’s body after she died, presumably looking for another host for its next stage of development. It could have been them, but fortunately the creatures seem to die if they don’t find one quickly. They wonder what the tiny little larvae will grow up to be…

Later, Owen and Martha brief the rest of the team in the boardroom. Owen shows them a molecular model of the drug they found in Marie‘s blood that she called reset. Owen says it’s like running a virus scan on a computer - the software works its way through the files and when it finds a Trojan or a virus it identifies it and deletes it. The reset does that to the human body, not just to viruses but also to harmful bacteria, toxins and even mutant cells. It’s the ultimate ’magic bullet’ and would be the greatest medical discovery in history - if it wasn’t for the fact that it comes bundled with a lethal alien parasite. The larvae need a healthy body until they’ve incubated, so the parasite ‘egg’ incorporates this magic bullet which resets the body back to its original factory settings. Jack asks what they know about the Pharm and Ianto tells them the public image seems innocent enough. It’s a partnership between the Government and a consortium of private pharmaceutical companies that researches and develops cutting edge biotechnology. Tosh reports that the Pharm’s IT systems are far more “cutting edge” than they need to be and they seem to have unrestricted security clearance, which means they’d have the capability to erase people’s medical records if they wanted to. The director of the institute is Dr Aaron Copley, one of the most respected research scientists in his field. He’s a graduate from Harvard University, did research at Cambridge and his last job was as Harvard Professor of Molecular Pharmacology before he came to the UK to set up the Pharm. Jack thinks he looks handsome and wonders if he’d like visitors.

The SUV heads towards a large stately home out in the country and pulls to a stop at a guarded checkpoint. The armed security guard approaches Jack, who simply says “Torchwood”. Surprisingly this seems to be enough for the guard, who allows the SUV to proceed up to the front of the house. Before long, Jack and Owen are invited into a large office to meet the director himself. Dr Copley flicks through the file they’ve presented to him, but then he apologises to them for what must be a wasted journey. He claims that none of the faces of the victims mean anything to him and if they’d been trial subjects at the clinic he’d have met them personally. Jack spreads the photographs out across the desk and tells Copley that two of the victims were murdered and the third victim named this clinic shortly before she died from a parasitic infection of alien origin. Copley says that’s preposterous. Owen tries a different tack and tells Dr Copley that his published works on immunology proved to be very helpful when he was writing his own thesis. He then suggests that if a drug did exist that could restore a human body back to its factory settings, surely Dr Copley would know about it? Copley agrees, but says nothing like that does exist. Jack openly accuses Copley of lying, so Copley brings the interview to an end and says he’s already late for his next appointment. Jack proposes looking around the complex and Copley says he’ll be delighted to arrange a standard PR tour, but warns that their research suites are classified and therefore out of bounds. Jack insists that Torchwood can go where they like, but Copley says that’s not the case here and they’ll have to take it up with Whitehall if they’re not happy. Jack says he had a bad experience with a politician recently, so he tends not to listen to Whitehall any more. Copley arranges for someone to escort Jack and Owen back to their vehicle.

As they leave the clinic, Jack and Owen agree that Dr Copley was a little touchy. They need to turn this place over so they decide to abandon the polite approach and try a different tactic. Jack takes some measurements from a device on his wrist-strap and says this clinic has the highest concentration of alien lifeforms this side of the Rift.

Tosh continues her attempts to hack into the Pharm’s computer system, but her access is continually denied. She warns Jack that if she carries on with her brute-force approach they’ll know they’re under attack. Jack insists that he needs a way in there, but fortunately Ianto has had more luck with the other research they were doing. He’s found a website that’s advertising for new volunteers for the Pharm’s clinical trials. Gwen suggests one of them go in undercover, but Jack says they don’t know enough about the place yet and too many things could go wrong. Martha suggests they put in a medic as they would know what to look for. Jack realises she’s volunteering herself and he refuses, but she reminds him that she’s been in much worse places. He eventually agrees, but Owen speaks to Jack privately and tells him he can’t just send Martha in like this. Jack assures him that she’s more than capable and he’d rely on her if the world was ending. In fact, he already has.

Ianto shows Martha the blueprints for the buildings that make up the Pharm clinic. Together with the observations made by Jack and Owen, they’re able to identify the location of Dr Copley’s office, the medical research suites and the accommodation for the clinical trial subjects. There are additional buildings at the rear which are restricted and are fenced off with armed security and Jack thinks this is where the readings of alien lifeforms were coming from. Ianto warns her not to go anywhere near there. All she needs to do is get herself accepted as a trial subject and not draw too much attention to herself. She assures him she knows how to be invisible. Once she’s in, her job is to gain access to the Pharm’s IT systems and close down the firewalls and security protocols so that Tosh can get into their files and find out what they‘re really up to. Then she needs to get out as soon as she can and not take any unnecessary risks. Martha teases Ianto by telling him Jack has asked her to get a UNIT cap for him. She asks him outright whether he and Jack are involved, and Ianto admits that they dabble. When she asks him what Jack’s dabbling is like, Ianto says it’s innovative and bordering on the avant-garde.

The group starts work on Martha’s cover story and Ianto provides her with fake ID under the name Samantha Jones. Tosh says communication is going to be tricky as the whole place is likely to be wired, but equally they want to monitor her at all times. Tosh produces a pair of contact lenses and when Martha puts them on, the others can see everything she sees through a monitor screen. Not only that, but the group can type in a message on their keyboard and the words appear before Martha’s eyes via the lenses. Tosh says the lenses are powered from body heat, so they’ll work all the time she’s wearing them. Owen teases Martha that they’ll be able to watch everything she does, so she says there are some things she’ll have to do with her eyes shut. In case of an emergency, they’ll be able to speak directly to her as the lenses can communicate with her sensory neural receptors, bypassing the auditory system. The signals won’t be intercepted because the alien technology exploits a solution to the EPR paradox. Martha understands all about quantum entanglement of remote particles, so that’s cool.

Much later, Martha has a blood test taken by the nurse at the Pharm clinic. The nurse explains that they’ll need two samples and one of them will be analysed overnight. In the Hub, the rest of the group are sitting around the monitor watching and listening to everything Martha experiences. The nurse asks ’Samantha’ to list the places she’s been to, so Martha reels off a list of places like North America, France, Germany and Australia. The nurse specifically asks about countries in the Third World - so Tosh instantly sends Martha an alert message warning her to be careful. Martha replies that she hasn’t and the nurse explains that they need to be wary of exotic tropical diseases in case there are any unexpected side effects. Martha says she’s also very conscious of health issues and the nurse reacts suspiciously. Tosh sends Martha another alert message, so Martha claims that her mum is a nurse. Dr Copley enters and the nurse introduces him to Martha. As Copley goes through Martha’s fake file, Jack sends her a message asking her to get close to him. She tells him she’s a student and says she really needs the cash, but Copley won’t commit himself yet as they get a lot of applicants. Jack tells Martha not to lose him, so as Copley turns to leave she announces that there’s something she held back that she really ought to have told them. She admits that she once had a hepatitis infection although she’s perfectly fine now. In the Hub Owen is getting worried as he thinks she’s bullshitting too much, but then Martha says that she knows the condition is incurable and that it stays in the blood. This is evidently what Dr Copley wanted to hear. He tells he this changes things entirely and they could really use a test subject with hepatitis. He asks her if she’s available to start straight away. Martha is delighted and says she’s already brought her overnight clothes, so Copley asks her to fill in a confidentiality form. In the Hub, everyone is relieved and impressed. Martha’s managed to get in alright, but they hope she won’t have any problems getting out again. Martha asks Copley what sort of drugs they’ll want her to take and he tells her they have various products at the clinical trials stage and she’ll get a full briefing once they’ve finalised her programme. Jack and the others watch as Martha is escorted to her room. On the way, they take in the layout of the clinic.

Later that night, Tosh chats with Owen and comments on how well he and Martha seem to be getting on. They look at a photograph of Martha and Tosh says she’s beautiful, and although Owen acts as if he’s not given the matter any thought, Tosh knows he‘s not telling the truth. And with Martha being a doctor too, Tosh thinks they’d be a perfect match. Owen dismisses the idea and says Martha’s only interested in work and in any case he thinks Jack would have his kneecaps if he tried anything on with her. He asks Tosh what happened to the pool tournament she said she was organising, but she admits she was really just trying to get him to go on a date with her. Owen looks surprised and Tosh is upset that he didn’t even realise that’s what she was doing. He asks her if she still wants to go on a date with him and she says she does, so he agrees. Tosh can’t believe it and thinks he’s being sarcastic or just polite, but he promises he isn’t. He agrees to one date, just to see how it goes, but he warns her this isn’t going to stop him flirting with other people. She’s happy with the arrangements and they get back to work.

Martha lies on her bed in the clinic, waiting patiently. Eventually she gets up and opens the door cautiously. The corridor outside is empty so she starts exploring. At the Hub, the rest of the group watch her every move closely. Owen sends her a message, warning her to be careful. She nods, then makes her way downstairs. She finds her way to a door marked ’Administration’ which she remembers passing earlier. She tries the door, but it’s locked and there’s an entry coder on the wall. Tosh asks Martha to stand closer to it so it falls within the EPR field, then Tosh begins to de-code it using their computer system. Following Tosh’s instructions, Martha starts tapping in the numbers, but then she hears two security guards approaching from a nearby corridor. Trying not to panic, Martha types in more numbers, but the last one takes longer than expected and she only manages to get inside the room with a split second to spare. The guards appear at the end of the corridor, but see nothing amiss and walk on.

Inside the room, Martha goes over to the Pharm computer and begins to access it. Tosh will need to hack into it, but it’ll take too long to talk Martha through it so she arranges with Martha to give her remote control access so she can operate the system from the Hub. They’re successful and Tosh manages to search inside the database and call up details of the alien parasite’s lifecycle. They Pharm have named the creature the Mayfly and Martha thinks it’s beautiful, but Owen reminds her that it‘s lethal. Jack tells Martha that because they’ve got control of the system they can download it all and go through it at their leisure, so there’s no need for her to stay at the clinic.

Martha returns to the corridor and begins making her way out of the building while the team in the Hub start reading through all the covert sections of the clinic’s database. Suddenly an alarm sounds in the Pharm and a voice comes over the loudspeaker announcing a breakout in Zone A. All patrols are ordered to the area and advised to proceed with caution as the escaped creature is extremely dangerous. Martha looks down on the courtyard below and sees a group of armed guards heading away from her location. Owen sends Martha a message telling her to get to safety, so she crosses to the other side of the room and slides open a window, climbing down to ground level.

Gwen has tracked down a list of all the clinical trial subjects and not surprisingly it includes the names of all the murder victims. Attached to each name is a note reading EXECUTIVE ACTION: BD. On the staff database they find an entry for a member of the security team named Billy Davis, who has an ultra security rating and is authorised for ’executive action’. Jack recognises the old CIA terminology and says it refers to assassinations. The Pharm obviously employ their own hitman, which is odd for a medical research facility. They find another name that they don’t recognise - Elin Morgan - who lives in Cardiff and who’s also been marked for ’executive action’. She hasn’t been reported missing or dead yet so they may still be in time. Jack asks Gwen to take Ianto and track the woman down before it’s too late.

Instead of taking the opportunity to escape from the clinic, Martha has instead decided to follow the security team in the direction of the mysterious Zone A. There she finds the buildings are shielded by a huge fence with barbed wire on the top. Another team of guards race towards her, so she ducks down and takes cover behind some nearby bushes. The guards open the gate using another entry coder and they race in, then Martha runs over and sneaks in behind them before the gate closes. In the Hub, Jack and Owen are watching her and wondering what the hell she thinks she’s doing. If the Pharm has its own hitman, who knows what else they’re capable of?

Martha finds herself in an exterior courtyard at the back of a row of buildings. She can hear shouts and whistles from nearby, but there’s no one else within sight. Then she hears a strange growling and fluttering and she slowly turns to see what could be making it. In the Hub, the picture coming through Martha’s contact lenses disappears and Tosh thinks it was caused by a radiation surge. In the courtyard, Martha cries out in pain and quickly removes the burning contact lenses from her eyes. Then, to her horror, she sees a gigantic insect, the size of a man, fly into the courtyard and slowly move towards her. She backs away as the security team arrive, but she’s spotted and one of the guards fires a tranquilliser dart at her. As she collapses, the giant insect disappears behind the building and the guards chase after it. In the Hub, the group realise something has gone seriously wrong. They have to get Martha out of there, but the clinic is swamped in security and if they try storming in they’ll be putting her at risk. Jack reminds them that Martha has been in worse situations than this and can handle herself.

As Ianto and Gwen race to Elin Morgan’s flat in the SUV they do a check on Billy Davis’s mobile phone and discover he’s already en route and will be there in just three minutes. Ianto puts his foot down. Elin is asleep in her bed when the door opens silently and Billy creeps into the room, armed with a hypodermic syringe. As he leans over her bed, Elin wakes up for a moment and Billy sprays her in the face with something that knocks her out. Then he opens her eyelid and prepares to inject her with the ammonium hydroxide. Suddenly the door is kicked open and Gwen bursts in, ordering Billy at gunpoint to put his hands above his head. The assassin obliges and she quickly disarms him, then Ianto joins them and knocks Billy out with his stun gun. Ianto calls Jack to give him the good news.

Martha wakes up to find herself strapped to a table inside the clinic. She struggles against her bonds, but to no avail. Dr Copley enters the room with the nurse and Martha demands to know what’s going on. She insists that she’s here as a clinical volunteer and says they can’t treat her like this, but Copley reveals that they know she’s been lying to them. Martha claims she simply got bored stuck in her room and went for a walk, then she got attacked by the creature and assaulted by the guards. Copley tells Martha that she doesn’t owe any loyalty to Torchwood and says Jack Harkness has treated her in a criminally irresponsible way. Martha pretends not to know what they’re talking about, but Copley tells her not to bother. He says this isn’t an interrogation and Torchwood is irrelevant to them. In fact, they’ve done Copley a favour by bringing Martha to him as they’ve analysed her test results and they know she’s something special.

In the Hub, Billy Davis struggles to release himself from the seat into which he‘s been strapped. He complains that he’s suffering from really bad guts and he doesn’t know what’s causing it, but Jack suggests it’s probably his conscience. Billy refuses to confess so Jack decides to do things the hard way. Ianto bursts into the room with a ferocious Weevil attached to a long chain. Billy cries out in terror as Ianto lets go of the Weevil and it charges towards him. The chain extends to its maximum limit and the Weevil pulls short just inches from Billy’s face. The assassin begs for mercy and agrees immediately to talk, so Ianto uses his spray to force the Weevil back out of the room. Billy admits that he works for the Pharm and says they’ve been giving people the reset drug, but there were side effects and he had to get to them before any symptoms started showing.

Dr Copley asks Martha if she knows what lymphocytes are and she identifies them as part of the immune system. The nurse tells her that her lymphocytes are really quite extraordinary and no one’s ever seen anything like them before - at least, not in a human being. Martha says she doesn’t know what they’re talking about, but the nurse explains that her cells have mutated under the influence of a radiation that’s not found in the temporally stable environments found on Earth. Copley reveals that they’ve dealt with aliens before, but they’ve never come across anything as ‘exotic’ as her. They know she’s a human being who’s travelled in time and space. Whatever the cause, Martha has been left with a uniquely effective immune system, which is exactly what they need. Copley says they’re developing a drug that will change the world, wiping out incurable cancers, AIDS and all the other scourges of the human race. It will be a revolution in medicine and social welfare, but unfortunately it‘s still not perfect and Copley is curious to know what her immune system will make of those imperfections. The nurse attaches a drip into Martha’s arm and when she tries to resist, Copley reminds her that she ’volunteered’. The drug slowly begins to enter Martha’s body…

Jack tells Billy he’s going to help them get back inside the Pharm. Billy agrees, but then he starts to cry out in agony as his stomach pain increases. Jack and Owen become genuinely concerned - and then Billy screams as blood bursts from his mouth and he starts violently shaking. Jack needs him alive so Owen desperately searches for something to help. He finds the Singularity Scalpel and directs it at Billy. On the small screen he can see through Billy’s body and focuses on a huge insect thrashing around inside. Owen is sure he’s got the calibrations right this time, so he activates the device to destroy the creature - and suddenly Billy’s stomach explodes and everyone is covered in blood. The insect is still alive and it drops down to the ground where Owen carefully picks it up and lowers it into a canister which he then seals. He realises this could easily have been him or Martha if they’d breathed in any of the larvae back at the hospital.

Martha is unconscious on the table in the clinic but her body is thrashing around. The nurse reports that she gave her twice the critical dose which means the Mayfly larvae are now incubating inside her. It’s clear that Martha’s immune system is reacting and there seems to be quite a fight going on. Dr Copley tells the nurse to keep her nerve and reminds her that they’re making history.

Tosh asks Ianto what he’s done with Billy’s body. He hasn’t had time to dispose of it yet and Tosh thinks she has an idea how they can use it to get them into the Pharm. She shows Ianto what she means and he thinks she must be warped on the inside!

The SUV pulls up to the security checkpoint outside the Pharm complex. The Torchwood group are all crouched in the back out of sight and Billy seems to be driving the vehicle. The guard takes a cursory look at him, recognises him and lifts the barrier to let the SUV through. As soon as they’re out of his sight, the group emerge from cover. Billy’s hands have been strapped to the steering wheel and Tosh is using a remote control device to operate the vehicle with a small screen showing what‘s ahead. The car pulls to a halt by the front door and Billy slumps forward. Jack orders Tosh, Gwen and Ianto to check out Zone A while he and Owen go after Martha.

Dr Copley is watching a monitor which shows the larva growing inside Martha’s body. Jack and Owen burst into the room and order him to raise his hands above his head. He complies but tells them Martha has already survived the larvae stage and she’s the only person ever to do so. He’s discovered that the bugs practice sibling cannibalism and only the strongest individual is left inside her now. Unfortunately he has no idea what’s going to happen next. Owen examines Martha and orders Copley to put a stop to it, but he admits that he has no idea how to do that.

Tosh uses an alien device to decode the gate to Zone A and they head inside. They break into one of the buildings and find dozens of huge canisters with jars attached to them extracting various fluids. They hide as the nurse enters and replaces one of the jars, then they step forward with their guns drawn and order her to put her hands above her head. They look closer at one of the huge canisters and are horrified to see a Weevil imprisoned inside. The nurse reveals that this is what the Pharm is all about. They farm captive aliens for the exotic chemical products they metabolise. The Weevils, for example, produce chemicals used in pesticides and in a very powerful defoliant. But it’s not the Weevils that are going to clinch the Nobel Prize for them - that will be down to the success of their Mayfly experiment. She shows them another canister which has a huge man-sized insect inside. This is their feedstock for the reset drug. Given time, they hope to be able to develop it for human use and it’ll be bigger than penicillin. Ianto contacts Jack and tells him that Zone A is filled with dozens of such creatures, all being used for different experiments.

The insect inside Martha’s stomach is now so huge Owen can actually see it moving around. He knows it’s killing her but he doesn’t know what he can do. Jack tells him to stay calm, but the only thing Owen can think of that might help is the Singularity Scalpel. While he wrests with his conscience, Jack tells Ianto to arrange for a total shutdown of the facilities in Zone A. He accuses Copley of creating a torture chamber and of abusing the Mayflies and turning them into parasites, but the doctor refuses to debate medical ethics with him except to say that they did everything they could to limit the damage. He doesn’t feel bad about murdering their test subjects as they were going to die anyway. He believes they’re on the edge of the greatest discovery in history and that’s worth a few sacrifices. Jack tells him he’s closing the entire operation down and he has no intention of going through official channels. He says they’re in control of their entire IT system and the databanks are crashing already, wiping all their records. Copley accuses him of cyber-terrorism, but Jack hasn’t finished yet. He also plans to trip the fire, radiation and biohazard safety networks, causing the system to shut down and the sprinklers to activate. Most of the buildings will be sealed and flooded with inert gases. Copley points out that this will kill the aliens, but Jack reminds him they were dying anyway, but at least this way they‘ll be put out of their misery. Copley argues that he and Torchwood are actually on the same side, but Jack says combating aliens is one thing, but this is slavery and exploitation, little better than a war crime.

On the table Martha’s body begins to jerk violently. Owen calls Jack over for his help and while they’re both distracted, Dr Copley slips out of the room. Jack demands to know what Owen is planning and he explains that the only thing that will save Martha now is the Singularity Scalpel. He’s confident he knows how to operate it, but Jack knows it’s never worked before. Owen switches on the device and on the small screen he can see through Martha’s body as it focuses on the huge insect thrashing around inside. Jack pleads with Owen to promise him it’s safe, but it’s far too late now and the insect is only seconds away from bursting its way out of Martha’s stomach. Owen activates the device and Martha screams and sits bolt upright, a look of shock on her face. Then she passes out and falls back down onto the table. Her life signs on the medical scanner flatline for a second and they both fear the worst - then Martha starts coughing and begins to sit up. Jack rushes over and drags her off the table. They need to get away from here as quickly as they can.

Outside the stately home, Gwen and the others are escorting the nurse away at gunpoint. Owen and Jack go out to join them, supporting the weakened Martha. Owen tells her she needs a few days in bed, under his supervision of course, but she tells him she already has a boyfriend. Owen says he won’t have saved her life like he just did, but she says that in fact he has. Tosh has set up all the necessary shutdown procedures and all that’s needed now is a press of the button on her laptop. Jack orders her to do it and the entire facility begins to close down.