Sam Tilson is an agent with MI5.
More specifically he is attached to a little known and even less understood department inside that larger organization which goes by the name of the Office of Inconclusive Covert Enquiries, abbreviated ICE. That group is run by the "sharp-witted" and not-to-be-underestimated Anne Barnard, a woman who runs a tight ship and who is determined to get the mission accomplished. That mission is to look into cold cases. While in police work that means opening files of crimes that had never gotten resolved, in the case of ICE it means checking out matters which seem to have a conclusion but of which there is some reasonable doubt. It is the job of Barnard and her people to crack open dusty file boxes and take another look. What they find, though, is that there are often people, some quite powerful, who want those boxes left alone.
Tilson comes to MI5 from having a successful and meaningful career with the elite Special Boat Service. He was fully expecting to have stayed with that illustrious group but when a surprise diagnosis of Type 1 diabetes. No longer being able to head out on missions because he would at any moment need to take a shot of insulin, his time with the SBS was done. It hit him very hard. He was 29 years old, seemingly in excellent shape, proud to hold the rank of Captain and being the commanding officer of his squad.
Then a call came that there was an opening at MI5 and Tilson was reenergized - until he learned that the position was with this unusual department that no one, including himself, had ever heard of. His enthusiasm wanes quickly. Still, Tilson is a dedicated officer and if that is where he must go to continue to serve his country and stay feeling like he is contributing to its safety, that is where he will go.