In the face of ongoing IRA attacks, the British secret service MI5 begins recruiting civilian informants in the 1990s. In the English county of Shropshire, students Ian Preston (Rob Malone), Sophie Jones (Marisa Abela), and Mae Hansen (Freya Mavor) are recruited for espionage by Robert (James Norton), but one night they are forced to go into hiding unprepared. Nine years later, London lawyer Alice Archer (Gemma Arterton) falls in love with Robert, who now works as a car salesman. When Alice notices that Robert is hiding something from her, she sets her friend Phil (Julian Barratt), a private detective, on him. Phil discovers that Robert's identity is fake. Alice confronts Robert, but he regains her trust by revealing himself to be an MI5 officer. Soon, new inconsistencies about Robert's past emerge: Sophie Jones' parents accuse him of kidnapping their daughter. Robert sells Alice the accusation as a secret service test of her loyalty. Suddenly, Robert disappears with the assets from a joint account he and Alice share. He briefly returns to Sophie, who has believed for years that she is involved in a secret service mission invented by Robert. Meanwhile, Alice convinces police officer Sonny Chandra (Shazad Latif) to hunt down Robert. He already has a new victim in his clutches and no longer has any scruples.