In the 1990s, the Irish Republican Army carried out an extensive bombing campaign in England as part of their long-drawn effort for Irish reunification. As a response, the British security service, MI5, that is, started recruiting freelance spies from the common English masses in order to gather intelligence on those suspected of being members of the IRA. A young man is seen driving down to his agricultural college in Shropshire, in 1993. The news on the radio mentions that a student has recently been arrested from the college for being an active IRA member, as the agricultural institution has become an easy source of fertilizer for the terrorists, who make bombs out of it. The man we see works as a barman in the local pub along with being a student, but very soon, he is revealed to be a member of MI5's amateur freelance spy program. He now recruits three of his batch-mates and friends, a man named Ian and two women (Sophie and Mae), to be part of the same secret service, and trains them for a first big mission. On the day of the mission, which is to search for any incriminating evidence in the house of one of the Irish students, the group of amateur spies somehow manage to pull it off. However, only a few days later, things go terribly wrong as the man suddenly wakes up his three friends one night, saying that their identities have been compromised and that they have to leave the college immediately, and drive them away.Nine years later, a successful litigation lawyer named Alice Archer (Gemma Arterton) returns home from an office celebration when she is approached by a charming man trying to make an acquaintance, but Alice rudely brushes him off. This man is the same man we have earlier seen Robert Hansen (James Norton), and he is now seen working as a luxury car salesman. The next day, Alice visits the shop he works at and apologizes, and Robert offers to drive her to her office. He also asks her out on a date, to which Alice agrees, but the man never shows up, much to her annoyance. When he gets in touch, though, Robert says that he could not arrive because of his father's serious illness. Mostly charmed by his quick replies and slick attitude, all of which seem very sincere to her, Alice forgives him and continues to see him, also joining him on a short trip to a nearby lighthouse. The two soon start a romantic relationship. Alice notices a woman named Sophie (Marisa Abela) calling Robert's cell phone, which he pretends to be a call from his dad.Alice hires a private detective of sorts and checks up on her new lover's past to find out that nobody named Robert Hansen exists in police records. When Alice confronts the man over this, Robert admits that he has actually been using an alias so far. He reveals that he is a spy in MI5, currently working undercover as a car salesman as part of a very serious investigation, and warns Alice that she would be checked by his employers, who would try to test her loyalty towards him by telling her absurd stories about him. Alice agrees to continue their relationship and face the MI5's test if it ever comes her way.Although Alice stays happy with Robert in her life, she tells him how she is not exactly too happy in her professional life and would rather have a business of her own. Robert proposes starting a business of leasing luxury cars with Alice, as he feels his time as a spy is coming towards its end. Alice agrees, but is perhaps a bit taken aback when Robert tells her parents of this plan when she introduces them over a formal lunch. Around this time, the detective who Alice had earlier hired also tells her about a court case in the name of a certain Robert Hansen-Freegard, who he suspects to be the same man as Alice's boyfriend. Robert had been presented to the court on charges of continually stalking and harassing a woman named Julie Harper, and the court had found him innocent.Despite Robert's claims that his employers will try to test her trust towards her boyfriend, Alice cannot shake off an ill-feeling and meets with Julie Harper's mother Bridget, whose contact she had received from the detective. After meeting her and getting to know shocking facts about how Robert had made Julie borrow twenty thousand pounds from her parents, apparently to get married to him, which never actually took place, she confronts Robert regarding all this when they meet for lunch. Robert, instead, claims that Julie was his first ever girlfriend, and that her mother hardly knows the full truth-Julie had been an alcoholic, and Robert had tried to get her to a rehab for which she needed money, but she did not want to tell her parents about this ill-habit and instead borrowed it saying it was to get married. But the woman apparently decided otherwise right before her time in rehab began, and when Robert tried to approach her, Julie brought false charges of harassment and stalking against her erstwhile boyfriend. After revealing all this, Robert leaves the scene, insulting Alice on the way.Just then, Alice gets a concerned call from another man, visiting whom she hears another frightful story. This man and his wife claim that Robert Freegard kidnapped their daughter, Sophie, and coerced her into cutting all ties with them. Sophie is indeed the same young woman who Robert had recruited at his agricultural college earlier, and the parents explain how she had disappeared from the college one day and then only contacted them saying that she was safe and doing some important secretive job along with the other recruits. The girl then also constantly told her parents to hand her over the inheritance money that she was about to receive after their death, and the parents had ultimately given her a hefty sum of three hundred thousand pounds, which they now believe to have been taken over by Robert Freegard. Alice and Robert meet one last time as they spend a night at her house, and both pretend to apologize and make up with each other, while they have completely lost faith in each other by now. The next morning, Alice suddenly receives information that the joint account that she and Robert had opened for their business was now closed, with all the money withdrawn from it (a total of 120,000 pounds); all the money that Alice had put in. She immediately realizes that it is Robert who has stolen her money, and upon getting home, she finds out that her passport is missing. Understanding what is up, she calls the police, and an officer is assigned to her case.In the meantime, Robert Hansen-Freegard is shown with Sophie at present, in all of which the young woman blindly believes that she is working on covert freelance missions for MI5. At first, Robert keeps her at a hotel for months, where she has to work as a maintenance worker, but Sophie believes that she is there to keep track of the hotel's operational timings and guest lists, which she does diligently. Robert then tells her how she only has one last mission before being an official MI5 member, and that is to take on the identity of a different woman named Alice Archer. The man had already stolen Alice's passport by now and had replaced the original photograph on it with that of Sophie, with which he then walked into the bank to clear out the joint business account. As both members would have to be present to close the account, he had made Sophie pretend to be Alice and got his intentions fulfilled. It becomes clear that Robert makes use of Sophie in every way possible, sexually as well, and he then drops her off in a shady neighborhood, saying that she will have to lay low here for some time before she is called up for her recruitment into the security service. While Sophie somehow survives by eating spoiled food and sleeping on the streets, Robert instead charms a new woman, an American psychology academic named Jenny Jackson (Sarah Goldberg).With Jenny, too, Robert first grows very close, making her fall in love with him deeply, with the utmost faith in him, while he only looks for vulnerable spots to make use of. He tells Jenny that the MI5 could make use of someone like her, with a background in psychology, but the only thing that would stop her recruitment was the student debt she had in her name. For this, he made her call her parents and ask for this money, a total of eighty-thousand dollars, which he would obviously steal and run. By this time, Robert had also learned that Jenny takes a number of prescribed antidepressants, and the man cruelly takes away all her medicine, claiming that she would not need any of it as he would fix her and make her happy. While Jenny now has violent fits as her body reacts horribly to the absence of the medicine, Robert just simply waits around for her parents in the US to send the money.With the help of Sophie's parents, Alice gets to know of the male student Robert had taken along to college, Ian Preston, and she goes to meet him. Ian tells her about how Robert would always find some pressure point in his victims, which he would use to then exploit them. He reveals that he could escape his cruel company only because Robert himself had let him go, and he also speaks of the third student, a woman named Mae Hansen (Freya Mavor). Alice now convinces the police officer working on her complaint, Sonny Chandra (Shazad Latif), to look for someone named Mae Hansen, and when they finally track her down, more shocking revelations are made. Mae tells them that Robert is actually her husband, and they even have two daughters together, but the man mostly stays away from the house on what she calls business trips. Alice gets hold of a pocket-diary with phone numbers in it from Mae's house and tries calling all of them up. By this time, Sonny takes her along to the American embassy, where they have contacted the police over their concern for the safety of an American citizen. This citizen, Jenny Jackson, had suddenly dropped all contact with her parents in the US and then had called up asking for eighty thousand dollars, which she apparently needed with regards to an employment opportunity that her new boyfriend, Robert Freegard, was helping her with.Alice now directly works with the US embassy's special agent, Harland (Edwina Findley), and together, the three manage to get in touch with Sophie from the numbers in the diary, and convince her that they are the MI5 wanting to recruit her. They pick Sophie up after the woman believes their story and tells them her location, and now, with Sophie's help, they manage to track down a house which Robert had claimed to be his, in which he currently holds Jenny hostage.The three, along with Sophie, rushed to the house, suspecting that they would most definitely find Robert Freegard there. At the same time, Jenny's parents send the money over and call their daughter's phone to inform her. As Jenny now lies unconscious and extremely sick from the withdrawal effects, Robert receives the call and lies that she is taking a bath. He now prepares to leave, with no concern for the dying Jenny whatsoever, and at the very same time, Alice and the rest arrive in front of the house. Sonny and Harland go over to check the house when Robert steps out onto the driveway and stumbles upon Alice face-to-face. Robert once again begs for her forgiveness and tries to make her believe that she is the only woman he has ever loved. It is when the other two officials arrive that Robert tries to make an escape in his car but is stopped in his tracks as Alice's car rams into his. There had only been Sophie inside the second car, and it is revealed that it was she who took the steering wheel and ultimately stopped the man who had exploited and cheated her for years.Robert Freegard is arrested while Jenny is rescued in her unconscious state, from which she recovers at the hospital. Sophie, too, returns to her parents, much to their joy, and two years later, Alice is seen on the witness stand during Robert's court trial. The court indeed finds the man guilty of the charges, and he is sent to prison. While Alice, now a successful businesswoman leasing luxury cars to corporate clients, visits the lighthouse where Robert had once taken her, the convicted man once again cooks up a spy story while in prison. A security guard is seen asking him whether it is true that he is an undercover MI5 agent. Robert only makes a sly face and says that he cannot disclose such confidential information to anyone.