Caption: In the summer of 2012 an event was to occur that would change the United Kingdom forever.
A busy summer day in London, as people go about their business, or attend the Olympic Games. Unseen, below the ground, a clock ticks down. A nuclear explosion devastates London.
Voiceover: Code 9. A nuclear attack on London. In bomb terms the blast was relatively small, but the death toll wasn't. Over a hundred thousand incinerated and many more given a death sentence by radiation. For some people outside of the blast zone, life continued almost as normal, but after the attack? Nothing in our country would ever be the same again.
Television news reports bring us up to speed. After the attack, the Government relocated to Manchester. Operation Sassoon saw people evacuated from London and the South East to relocation centres in the North of England, while controversial new security measures were introduced across the country.
Caption: MI5 field offices were created, to infiltrate and destroy a new generation of terrorists.
A new kind of agent was recruited. Hannah, leader of Field Office 19, recruits Jez Cook, a former petty criminal; Rachel Harris, a police officer; Rob Sullivan, a medical student; Vik Kamath, an entrepeneur; and Kylie Roman, a psychology student.
Caption: 11 months on these new recruits are hitting the frontline...
At Field Office 19, Sector 1C, West Yorkshire, Leader Hannah is concerned about security arrangements ahead of a Prime Ministerial visit in 10 days' time. Rachel is on operation control. Kylie and Rob are undercover at Club H in Sector 3G, and Kylie has made contact with their target. Target's lackey follows Kylie to the toilets, where she is meeting Rob to hand over samples for DNA analysis, but they overpower him and escape from the club. Vik and Jez are undercover at a secured food supply compound in Sector 31B, carrying out surveillance on Flynn Dixon. They photograph him talking to a teenager in a hoodie. Dixon pulls a gun on them, but a fellow worker, Charlie Green, comes to their assistance, knocking a pile of boxes onto the target and allowing the three to escape.
Back at base, Hanna introduces Charlie as the final member of Field Office 19. Flashback: Charlie, a mathematician, is recruited by Hannah. Hannah explains that she asked Charlie to tail Vik and Jez.
The working day is over, so the six field officers repair to a club to unwind. Back at the office, Neil Mayhew, a former MI5 agent, calls on Hannah to offer her a job, but she's not interested.
Rachel joins Charlie on a rooftop, and they trade personal histories. Five of them go to the house that Hannah, Rachel, Rob and Vik share together, but Jez is out, visiting the Missing Persons Officer, still looking for the family he lost after the blast.
Next morning, Vik and Charlie are going to work. Charlie has forgotten his ID card, and is impressed when the officer at Checkpoint 17 on St Georges Square checks his details and calls him "sir".
Hannah briefs the team. The PM's visit has been brought forward to today. Kylie's target is identified as Theo Danakis, a known arms dealer. Flynn Dixon, the man at the warehouse, is believed to be organising a hit. One of Jez's informants has said that Jermain Lee, an assassin, is rumoured to be in town. Jez and Charlie meet the informant to get more information.
Kylie and Rob interrogate Danakis. He won't cooperate, until Rob shoots his big toe, when he reveals that he supplied Lee with an AK-47 sniper rifle. Charlie phones around take-away restaurants until he finds one that delivered a burger prepared to Lee's specifications.
Hannah briefs her boss, Sarah Yates, Director of Field Operations, but Yates is not impressed and decides not to cancel the visit.
The team get a positive location for Lee: a room in the Laurant Hotel on St Georges Square, next to the Field Office, and overlooking the route of the PM's convoy. The team take up position and secure the floor. Charlie sees Lee in position at the window, hours ahead of the visit. Hannah arrives on scene, and is shot. Charlie rushes to her side, followed by Rob, but the men are unable to assist and Hannah dies. Jez and Kylie go to arrest Lee, but he escapes from the hotel. As they give chase, Lee falls to his death.
Yates arrives at the Field Office, where Hannah tells her that they have confirmed that Hannah was the intended target, Yates is not impressed to discover that the assassin has died, and that the team has no idea who hired him. The Prime Ministerial visit has been cancelled, and an officer from Central Command will come over to lead the investigation into the shooting.
Charlie protests that this will be a waste of time. He says that Field Office 19 can deal, and asks for five-and-a-half hours. Yates agrees.
Charlie tells Rob that Yates would not have agreed to a longer time frame. By asking for an odd half-hour, he put her on the back foot.
The team track the broker, Dixon, to the Wellstone Clinic. Rob and Rachel, disguised as a doctor and nurse, anaesthetize Dixon and bring him in for interrogation. When Dixon wakes, Rob pretends to be Lee's handler, and demands the balance of the fee and compensation for Lee's death. Rob gives Dixon a mobile phone to use to call him.
A tracking device in the phone allows the team to follow Dixon to the man who paid him to organize the hit: Neil Mayhew. When Mayhew realizes that Dixon has been set up, he flees the scene, and phones to request an intercept team to help him escape MI5. Kylie and Rob pursue Mayhew to Longford Arcade, and are joined by Charlie and Rachel, then Jez and Vik. The field officers close in on Mayhew's position, and pin him down in an empty shop. When the agents close in on him, they find him on the balcony outside, with a knife in his back.
Charlie reports developments to Yates. She is aware of a history between Hannah and Mayhew, and suggests that the killing may have been personal. With Mayhew's death, they may never know, but Yates does intend to find out. Meanwhile, there is a vacancy in Field Office 19. Charlie says that Rachel is the obvious choice, but Yates gives the job to Charlie. She was impressed that he alone stood up to her. She suggests putting "interim" in front of the job title, if that would make Charlie feel more comfortable.
The team has mixed reactions to Charlie's appointment. Charlie wants to find out who killed Hannah, and why. Flynn is dead, apparently having killed himself, so there are no available lines of enquiry.
That night, Sean, who identifies himself as an old friend of Hannah, approaches Charlie and gives him a package, saying that Hannah asked him to give it to Charlie if she died. Charlie opens the package, which contains a memory stick.