The Human Factor is the story of Maurice Castle, an MI5 agent married to a black South African woman. When information leaks are discovered in Castle's department, suspicion falls on his coworker, a young man named Arthur Davis. The bureau arranges Arthur's death, although they have no definite proof of his double-agency. It turns out that Castle is the double agent, turned during his time in Africa because of a desire to help his wife's people escape the scourge of apartheid.
The Human Factor's MI5 department is the anti-Bond. Castle and his coworkers feel about their jobs the same way a salesman or a hatmaker might feel about theirs: it's money and it keeps the lights on. No one, not even the high ranking officials who eventually seal Castle's fate, see their job as a great national responsibility. They all function as their own tiny piece of the whole, comparing their positions to “boxes” which they need never leave and no one else need enter.