The disappearance of a Foreign Office official appears to explain the deaths of a number of Russian agents working for British Intelligence. An apprehensive Sam is also now working at MI5 HQ while helping her husband Adam, who is standing as a Labour Party candidate at the Peckham by-election. While on the campaign trail, Adam meets a woman who asks for his help in finding her daughter Evelyn who has been missing for three days. Back at MI5, Sam notices documents referring to Evelyn Greene, the missing woman from her husband's potential constituency. But as Foyle continues with his enquiries, the wall of silence surrounding a mysterious military facility leads him to question the motives of his own colleagues.