Former British secret agent Harry Palmer is drawn back into the world of espionage when he receives a mysterious package containing a large sum of money and instructions to deliver a thermos flask to Helsinki. Despite his initial reluctance, Palmer contacts his old superior, Colonel Ross, who advises him to follow through with the instructions. Upon delivering the package, Palmer becomes embroiled in a complex plot involving multiple intelligence agencies and political conspiracies. He meets an American operative named Leo Newbigen, who brings him into the fold of an elaborate scheme orchestrated by a reclusive Texas billionaire named General Midwinter. Midwinter's grand vision is fueled by anti-communist sentiments and revolutionary zeal, leading him to invest in a private spy network using an advanced supercomputer.

As Palmer delves deeper into the clandestine operations, he encounters a host of colorful characters and dangerous situations, including subterfuge from fellow spies and violent engagements. The narrative twists and turns through various European locales, unveiling layers of deceit and hidden agendas. Amidst the subplots of betrayal and loyalty, Palmer must navigate through complex webs of intrigue to understand the true objectives of those involved. The tension escalates in this high-stakes game of espionage, where not everything is as it seems, ultimately leading Palmer towards startling revelations that challenge his perceptions and ethical boundaries.