Sarah has arranged to meet Josh and Nat at her apartment, but something else arrives as well -- a glass cage with a rat inside and a note attached. When Josh removes the note, the cage seals itself and fills with nerve gas, killing the rat quickly but painfully. The note, signed “an old acquaintance,” informs Sarah that the same will happen to London within 24 hours unless Sarah stops it...

Sarah phones up her civil servant friend Claudia Coster, but Claudia isn’t free to talk -- she’s in a meeting with a certain Mr Harris, discussing the Biogard incident. After their meeting, Harris personally contacts James Carver, the man who jostled the minister outside Biogard. Carver, a former soldier and staunch supporter of veteran’s rights, is confused and disoriented, but he listens as Harris explains what he has in mind. Carver seeks compensation for British soldiers used as medical guinea pigs without their knowledge, but his demands have been ignored, and his crusade has cost him his home, job and marriage while the original tests have cost him his health. But if he does as Harris suggests, he will never be ignored again...

Nat investigates the incident at Biogard, suspecting a connection, and learns that several lab rats were stolen in a recent break-in. She also identifies James Carver, whom Sarah recognises from an interview she once conducted; since then, however, he’s lost a lot of weight and hair. Josh decides to investigate Biogard undercover, while Sarah meets privately with Claudia to pass on the warning. Ellie arrives shortly afterwards to meet with Josh, only to learn she’s just missed him. She complains to Nat about the security thugs who evicted her group from the tube station without even letting them collect their belongings, and leaves a message for Josh to meet her at the Serpentine Gallery at 3:00. Meanwhile, Josh visits Biogard, posing as a reporter, and is given a tour by the company’s head of security -- Philip Harris.

Sarah and Claudia meet in Pall Mall, and Sarah hands over the cage with the dead rat and the note which came with it. Claudia admits to Sarah that the break-in at Biogard is more serious than anyone’s let on -- it appears that a tray of sarin pellets may have been stolen as well. Claudia agrees to analyse the cage’s contents and investigate James Carver’s connection to the company. Sarah returns to her flat, where Nat provides her with terrifying statistics about the sarin gas attack in the Japanese subway and the presence of suspected terrorist cells in London. Josh arrives and confirms that Biogard has been developing antidotes for bioweapons, and that it’s possible some of their test samples were stolen. Nat wants to go to the police, but Sarah admits that she’s handed over their only evidence. Josh is furious and storms off to meet Ellie, accusing Sarah of making decisions without consulting her friends. Nat admits that she too feels left out sometimes, and Sarah, irritated, starts to follow Josh -- only to run into James Carver at the doorstep ...

Josh arrives at the Serpentine late, to find that Ellie has given up waiting for him. She’s left him a text message on his mobile phone, directing him to the old Brampton Road station on the Piccadilly line, where she’s gone to collect the gear she left behind. He makes his way to the tube station, breaking through a side door, only to find Ellie’s bag lying abandoned on the stairs while muffled cries for help come from the platform. He investigates to find that Ellie has been locked inside a larger version of the same glass cage the dead rat was delivered in -- and as he tries to get her out, someone very familiar shows up and shoots him...

Carver explains that he saw Sarah in St James’ Park and recognised her from the interview she conducted with him. Prone to fits of violent temper, a wandering mind, blackouts and memory loss, he first asks her for help and then reveals that he’s carrying a pellet of sarin gas -- and that he’s planning to release a whole tray of pellets in the Underground tomorrow at rush hour. He then excuses himself to use the bathroom. Nat, terrified, insists upon calling the police, but moments after she hangs up, Carver bursts out of the bathroom in a paranoid rage and accuses her of doing just that. As Sarah tries to talk him down, Claudia calls Sarah back. Furious and paranoid, Carver seizes the phone and demands to know who’s calling. Claudia knows better than to lie; according to Carver’s medical history he’s suffering from a brain tumour which makes him very aggressive and susceptible to suggestion. When Carver learns that Claudia works for the government, he gives her an ultimatum: he wants to meet with the PM to discuss compensation for British soldiers given unsafe vaccines against their will, and if he doesn’t hear an answer on the 6:00 news tomorrow morning, he will release sarin gas in the Underground.

Carver’s mood plummets after he disconnects, but just as it seems Sarah is about to talk sense into him, the police arrive. Furious, Carver threatens to release the sarin pellet, but pulls himself together and flees, telling Sarah sadly to remember him as he was. Infuriated, Sarah leaves Nat to explain herself to the police while she hides on the fire escape and calls back Claudia. Claudia is trying to get Carver’s warning to her superiors, and advises Sarah to drop by her flat later. Nat gets rid of the police, telling them that the call was a mistake, but Sarah is furious with her for making the call in the first place and thus driving away their only lead. Nevertheless, Nat continues her research, and unearths the terrifying results of government experiments into possible germ warfare on the Underground; the full results of the tests were never made public for fear of causing a panic. Disturbed, Sarah sets off to speak with Claudia, leaving Nat to hack into Biogard’s staff database and try to find out who could be responsible for the break-in if it was indeed an inside job.

Josh wakes up to find that, like Ellie, he’s been trapped inside a glass cage on the station platform. Harris merely shot him with a tranquilliser dart, and he’s been waiting outside for Josh to wake up just to make his position perfectly clear. Like the break-in, which Harris staged himself in order to get the sarin, this is all part of a complex game his employers are playing with Sarah. Harris places a sarin pellet in a small chamber atop the cages; if Ellie and Josh try to escape, they’ll be dead in less than a minute. Harris leaves them to spend an uncomfortable night alone in the cages; Josh still has his mobile phone, but he can’t get a signal down here. He’s desperately worried, but not just for Sarah -- if Harris is willing to kill Sarah’s friends to make a point, then what’s going to happen to Nat? Elsewhere, Carver admits that he’s having doubts, but Harris pushes him to go through with the plan in order to make his suffering mean something.

Sarah arrives at Claudia’s flat only to find that her friend has just been shot. Claudia dies warning Sarah to get away, but it’s too late -- Detective Inspector Morrison arrives to arrest Sarah, having received an anonymous phone call informing him that she’d killed Claudia. Sarah is taken in for questioning, and eventually must tell Morrison everything; unfortunately, Nat’s earlier claim that her call was a false alarm now plays against her. Sarah spends the entire night in the interrogation room, desperately aware that time is running out for the commuters of London...

Natalie works through the night, nearly falling asleep in her wheelchair -- but she snaps fully awake when Harris uses Josh’s keys to enter the flat. Harris smashes the computer, informing the terrified Nat that he and his employers always know where Sarah is to be found, no matter how hard she tries to hide. Harris tips Nat out of her wheelchair, takes it out of the flat -- and returns with a bomb. Harris’ employees want Sarah to suffer, and they’re going to start by killing her friend. Harris sets the bomb’s timer and departs, leaving Natalie only 30 minutes to crawl to safety. She begins to pull herself out of the flat, furious and refusing to let Harris make her a victim. Meanwhile, Carver phones up Claudia Coster’s office and leaves a message on her voice mail. It’s 6:00, his demands have not been met, and he wasn’t bluffing.

Claudia’s neighbours confirm seeing Sarah arrive after they heard the shots, and Morrison thus releases her, but advises her to keep her wild story about sarin gas to herself. She storms out of the police station and immediately receives a phone call from Harris. It’s time to turn the screws. Natalie is trapped in Sarah’s flat with a bomb, and Carver about to release sarin gas into the Underground. The police won’t believe Sarah if she warns them, and if she tries to do so anyway, Harris will detonate the bomb immediately. Sarah must choose between saving her friend and the lives of millions -- and she can’t even be sure whether Carver will carry out his murderous plan. Time is running out, and as rush hour has just begun, Sarah is simply unable to get back home and then to the disused platform in time. She tries to call Natalie, but the mobile phone is still in Nat’s car. Sarah must make a terrible choice...

The disoriented Carver stumbles into the Brampton Road station, where he finds Josh and Ellie and Josh talks him into releasing them. Ellie sets off to warn the police, while Josh remains behind, intending to get a piece of Mr Harris when he returns. But he soon learns that it’s Carver who has the sarin pellets, literally strapped to his body. Josh tries to talk sense into him, but Carver is sick of listening to other people’s lies, and he won’t let anyone stop him; he’s going to make his presence known, by throwing the pellets in front of the next tube train to pass through the station.

Sarah arrives at the station, and also tries to speak sense to Carver, trying to get him to understand that he’s being used as a pawn just to get to Sarah. Harris then joins the debate; he’s watching everything unfold on closed circuit television, and he uses the station’s public address system to speak to Carver, urging him to go through with it. A train is approaching, and if Carver throws the pellets in front of it, he will kill thousands, perhaps millions of innocent people. Forced to choose, Carver finally realises he can’t go through with it, and he puts down the sarin pellets, asks Sarah to forgive him, and leaps in front of the train as it passes through the station. The city is safe... but has Nat paid the ultimate price?

The good news is, Nat is alive; though she scraped her hands raw pulling herself out of the flat, Harris underestimated her and left the wheelchair discarded just outside. The bad news is, the close call has shaken her badly -- but she’s even more shaken to understand that when Sarah had to choose, she didn’t choose to save Nat. Intellectually, Nat knows that it was the right decision, but emotionally she still feels betrayed, and she has to take some time off. It seems Harris has succeeded in driving a wedge between Sarah and her friends. Josh assures Sarah that he’s still on her side for the long haul, and suggests that they take a holiday, but Sarah is beginning to believe that to save her friends she must face her enemies alone.