Treason follows Adam Lawrence, trained and groomed by MI6, whose career seems set. But when the past catches up with him in the form of Kara, a Russian spy with whom he shares a complicated past, he is forced to question everything and everyone in his life. A triangular relationship forms between Kara, Adam and his wife, Maddy; three people who are trying to expose each other's secrets, navigate political and diplomatic relationships, whilst hanging onto their personal lives, and those they love most.