A series of seemingly professional murders in Cardiff prompts Jack to call in UNIT medical specialist Martha Jones for help. The Torchwood team are surprised and curious about Dr. Jones' relationship with Jack, as they still do not know much about his past. Martha informs him that there have been similar murders throughout the UK. Both Owen and Jack flirt with Martha, causing Gwen to show some jealousy, though she and Martha end up clicking together as great friends.
Jack is more open and light- hearted when Martha arrives, dropping hints about their travels with the Doctor. When the two sit down for a brief catch-up, Jack says to Martha that she should have contacted him if she was looking for work. Martha replies that she wasn't and that UNIT rang her out of the blue. They informed her that she was what they needed and came recommended from an 'impeccable source'. Both Jack and Martha conclude that it must have been the Doctor, as no one else knew about Martha's time travelling experiences. Jack says thoughtfully that perhaps the Doctor thought he owed Martha a favour...they all did, which causes Martha to ponder about the possibility.
As the Torchwood team continue to investigate the murders, they discover that one of the victims had been cured of diabetes and another of HIV. With this information they trace the killings back to a medical centre named The Pharm. One woman had been attacked but survived and was in a hospital. She has a seizure after admittance and fly-like creatures rise out of her mouth and try to enter Owen and Martha, but die before they get the chance. Owen and Jack go to the Pharm and talk to the Director, Professor Aaron Copley, but are unsuccessful in gaining access to any areas of the facility.
After searching for ways of infiltrating the Pharm, the team discover that the facility recruits people for human trial purposes. Jack concludes that it is far too dangerous to send one of their own in undercover but Martha suggests that if a medic with the appropriate knowledge went in there; it would be less dangerous. After an initial protest from Jack and a silent one from Owen, Martha convinces Jack to let her go in undercover as she's 'been in worse situations'. Owen follows Jack and begs him not to let her go in there, to which Jack reassures him that he'd rely on Martha if the world was ending...and he has.
Ianto sets Martha up with her instructions and gear, and a short conversation about his and Jack's relationship ensues. Toshiko fits Martha with contact lenses that act as cameras, so that the team can monitor her while she is inside. Martha then goes undercover as Samantha Jones, a hepatitis sufferer, in order to investigate the centre's real purpose.
Meanwhile back at the Hub, Tosh and Owen are keeping watch over Martha in the Pharm. Tosh remarks upon seeing Owen gazing at the picture of Martha's UNIT badge, about their quickly developing friendship and that she and Owen are a perfect match, both of them being doctors. She goes onto to point out Martha's obvious beauty and shares a laugh with Owen, who tries to feign ignorance. Owen remarks that the flirting is harmless seeing as Martha is only interested in work and that if he tried anything "Jack would have his kneecaps". This seems to cheer Tosh up somewhat, and Owen asks her about the pool tournament (First mentioned in 'Meat') she was supposedly organising; to which Tosh says that it was supposed to be a date. Owen looks taken aback and asks Tosh if she was still interested, to which Tosh eagerly says yes. She explains that if they got to know each other outside of work, something could eventuate out of it and Owen casually agrees to one date, which excites Toshiko immensely. He does point out that he wouldn't stop flirting with people just because of their situation and Tosh tells him that he can be 'King Of Flirts'. The conversation disintegrates and they change topic and watch over Martha again.
While inside the Pharm, Martha discovers that the facility is responsible for the drug "Reset", which is what cured the previously incurable diseases. But the problem with Reset is that it works by releasing alien parasites, called Mayflies, into the human bloodstream - the Mayfly larvae proceed to "reset" the human body, eliminating even incurable diseases such as HIV. The downside to the drug is that the Mayfly larvae eventually hatch and eat the person from the inside.
Martha is caught snooping around the mayfly laboratories and the medical director examines her and states that her white blood cells have mutated due to "travel in time and space'. He questions Martha about her alliance with Torchwood, and about her travels in time and space, all the while Martha acts as if she doesn't know what they are talking about. He is curious to see how her immune system will react to Reset and injects her with it.
When Gwen and Ianto track down the Pharm-hired assassin who is carrying out the murders, arrest him and take him to the Hub, Jack and Owen find that he has a mayfly in him as well during questioning. Jack needs the assassin alive and Owen attempts to use an alien artifact, the singularity scalpel, to vapourise the mayfly within his body. He screws up and the fly bursts out of the assassin's stomach instead. Toshiko concocts a program that would get them into the Pharm by controlling the car from the back while the dead assassin was strapped in the driver's seat.
Once they get inside the Pharm, the team split up: Tosh, Gwen and Ianto are sent to find the 'research labs', while Owen and Jack find the Director and Martha. Tosh, Gwen and Ianto discover the 'labs' to find that alien life-forms are being held captive and virtually being tortured. By the time Jack and Owen reach Martha, the Reset drug has been injected and the mayflies fought each other off until one mayfly remains inside. While Jack tells Prof. Copeley that his team are shutting the Pharm down as a result of the damage they are doing, Owen tends to Martha, using the singularity scalpel to extract the mayfly from her body safely. The trio exit the Pharm before it reaches complete shut down and meet up with the other three by the 4WD they came in. Owen, who is supporting Martha as they walk; continues to flirt with her (i.e. by suggesting she should spend a few days in bed, hopefully under his supervision etc), at which point Martha says that she already has a boyfriend. Owen asks if he's ever saved her life like he just did, to which Martha replies that he had.
As the team get ready to leave and Owen is ushering Martha towards the car, Prof. Copley returns and points a gun at Martha, telling Jack and the others that he wouldn't just stand by and watch them ruin his work and let them get away with it. Owen, who had formerly admired the Professor, steps carefully in front of Martha and attempts to reason with the Professor, claiming they are both rational men and that he doesn't really want to shoot. Professor Copley shoots Owen in the chest, to which Jack retaliates and shoots the Professor in the head, killing him instantly. Martha, Jack and Toshiko run over to Owen, who lies on the ground, bleeding. While Martha gets out medical supplies, Jack holds Owen's hand and reassures him he'll be alright while Toshiko becomes a little hysterical as she kneels beside Owen's dying body. Martha and Jack try to get him to focus, while Martha attempts to treat Owen's wound by injecting a solution near his heart. Toshiko, who is visibly distraught looks over him as a look of peace comes over Owen's face and Martha says tearfully that he is dead.
John Barrowman – Captain Jack Harkness
Eve Myles – Gwen Cooper
Burn Gorman – Owen Harper
Naoko Mori – Toshiko Sato
Gareth David-Lloyd – Ianto Jones
Freema Agyeman – Martha Jones
Alan Dale – Aaron Copley
Jacqueline Boatswain – Plummer
Jan Anderson – Marie Thomas
Rhodri Miles – Billy Davis
Michael Sewell – Mike
John Samuel Worsey – Policeman
Paul Kasey (uncredited) – Weevil
Writer J.C. Wilsher
Director Ashley Way
Script editor Brian Minchin
Producer Richard Stokes
Chris Chibnall (co-producer)
Executive producer(s) Russell T. Davies
Production code 2.6
Series Series 2
Length 50 mins
Originally broadcast 13 February 2008