The Human Factor by Graham Greene is a novel about Maurice Castle. He is 62 years and does office work for a British intelligence agency. He is married to Sarah, who comes from Africa. They have a son called Sam. His responsibilities at work entail liaising with British agents in South Africa. He works together with Arthur Davis in the same office. Previously, Castle had been posted in South Africa, where he was a field agent. When Castle goes out for lunch on a normal working day, he bumps into Colonel Daintry, who is now in charge of security. He requests to check Castle’s bag to ensure that he is not carrying top secret documents outside the building.

The intelligence agency has been suspecting that there is a leak in Castle’s subdivision, which comprises of only two people. As a result, security checks are conducted randomly, which explains why Castle was searched. Daintry and his superiors plan to kill the person responsible for the security breach to protect the agency from public humiliation. With the help of Doctor Percival, they decide to poison the culprit. After spending a brief time with him, Percival is convinced that Arthur is the leak.