“He had a long thin nose, a moustache like flock wallpaper, sparse, carefully combed hair, and the complexion of a Hovis loaf."

An enjoyable but bewildering and confusing book. Deighton withholds almost all the information needed to make sense of the plot. Even a basic structure of the story is difficult to discern. There are endless twists and turns, digressions and movements in place and time. I'm sure this was super cool back in the 1960s with its leisurely and convoluted meandering.

Here's my version of what I thought the plot was about:
[Deighton frames the novel as the unnamed protagonist delivering his personal report on "the IPCRESS affair" to the Minister of Defence. This makes the novel itself the 'IPCRESS File' of the title. The events begin soon after his transfer from military intelligence to WOOC(P). This is a small civilian intelligence agency reporting to the British Cabinet under a man named Dalby. An intelligence broker code-named "Jay" may be behind a series of kidnappings of influential British VIPs. The intention of this is to sell them to the Soviets. The protagonist is soon assigned to meet with Jay. This is to secure the release of "Raven", a high-ranking scientist and his latest target. After meeting Jay at a sleazy Soho strip club to negotiate Raven's release, the protagonist is abandoned. He investigates his surroundings and discovers Raven's unconscious body in a back room and fails to rescue him.

WOOC(P) receives intelligence of Raven's transfer to the Soviets in Beirut. Dalby organises a rescue mission with the protagonist participating. The protagonist is a lookout while Dalby kills Raven's captors and rescues him. He kills the occupants of a car which arrives on the scene to maintain the cover of the operation. He believes them to be operatives working for Jay. They instead turn out to be members of ONI. The operation is otherwise a success and Raven is recovered, but the investigation into Jay continues. Dalby disappears, going undercover, leaving the protagonist in charge of WOOC(P). At this point the protagonist's former superior from military intelligence, Colonel Ross, approaches the him. He offers to sell him confidential information related to the affair. The protagonist rejects the offer in disgust, but begins to second-guess himself.
Carswell, a statistician assigned to the matter notes bizarre links between the kidnap victims. A break appears with the arrest of Housemartin, one of Jay's high-ranking operatives. He's been impersonating a police officer. The protagonist and Murray, another operative assigned to the case, arrive at the police station but they discover Housemartin's murder. Information from the arrest enables WOOC(P) and the police to storm one of Jay's safe-houses. This find that this is abandoned. To help with the administration of the department, the protagonist is assigned gets an assistant, Jean. Jean is a beautiful young woman and he begins to develop romantic feelings towards her. Dalby re-emerges, and reveals intelligence suggesting that Jay's operations will interfere with an American neutron bomb test in the Pacific.

Dalby, Jean and the protagonist arrive at the test site as British observers. While there the protagonist learns from an old friend, Barney, that the Americans suspect him of being a double-agent due to the deaths of the CIA operatives in Beirut. Jean reveals to the protagonist that Dalby has left been visiting an abandoned Japanese bunker on the island. Soon after, Barney is killed in suspicious circumstances. While following Dalby to the scene the protagonist is present at the sabotage of bomb test site. This sets back the bomb test plus a military police officer dies. The protagonist is arrested by the Americans and interrogated. Then he is transferred to Hungary on suspicion of being a Soviet agent. There, he is drugged and subject to days of psychological and physical torture. He almost cracks before managing to escape. He discovers that he is in fact back in London. The protagonist takes refuge with Charlie Cavendish. Charlie is the father of a friend killed towards the end of the Second World War. He then tries to re-establish contact with WOOC(P) while evading arrest for treason. Charlie is killed by Jay's operatives, forcing the protagonist on the run. He approaches Dalby at his home, but discovers Dalby meeting with Murray, Jay and another of Jay's operatives. This confirms the protagonist's suspicions that Dalby is in fact the traitor.

Murry discovers the protagonist, who reveals himself to be an undercover operative from military intelligence also investigating Dalby. The protagonist escapes, but is soon captured by Jay's operatives and taken to meet Jay. But military intelligence follows them, and Jay and Dalby are arrested by Colonel Ross. The protagonist reveals to Jean that Jay and Dalby were using a process called "Induction of Psycho-neuroses by Conditioned Reflex with Stress" (IPCRESS) to brainwash the VIPs into loyalty to the Soviet Union. They had also unsuccessfully attempted to subject the protagonist to this too. The seemingly irrelevant links that Carswell had discovered were in fact indicators of the personality traits that Jay had used to determine which VIPs would succumb to the process. Dalby was the one who had sabotaged the American bomb test, as part of Jay and Dalby's efforts to frame the protagonist. Colonel Ross reveals that his attempt to sell information to the protagonist had been a test of his loyalty. The protagonist had passed by rejecting it. The novel ends with the protagonist concluding his report to the Minister. It reveals that Jay has turned and began working for the British. It also reveals the execution of Dalby with his death covered up as a car accident.(hide spoiler)]
This is one of those instances where the film is better than the book. It makes more sense for starters, plus Michael Caine puts in an excellent turn as Harry Palmer.

Buy a copy HERE.