4 ideas
The narrator is arrested at a US atomic test on a Pacific atoll, then held in what he is led to believe is a prison in Hungary. There he is subjected to sleep deprivation, disorientation and conditioning. When he escapes, he finds he has been in London all along, and the foreign setting was staged to make him believe he was cut off from home and help.
IPCRESS stands for Induction of Psycho-neuroses by Conditioned Reflex under Stress. First the subject is broken down with isolation, sleeplessness, false signals about time and place, and physical strain. Then new reflexes are installed that can later be triggered by a cue. The method works because stress removes the person's frame of reference, and that frame is what normally lets someone resist suggestion.
The agent's daily fight is less against Soviet opponents than against disputed expense claims, grudging clerks and rival departments guarding their territory. Intelligence work runs on the same small office politics as any civil service. Energy spent on internal credit and accounting often shapes an operation more than the external enemy does.
The grammar-school, working-class narrator is treated as a useful but suspect subordinate by public-school superiors. The Establishment trusts people who share its manners and codes. That same trust lets a smooth, well-born superior operate as a traitor, while suspicion falls on the outsider who is actually loyal.