In pre-revolutionary Cuba, a British expatriate named Jim Wormold finds his life as an unsuccessful vacuum cleaner salesman spiraling into financial distress. His wife has left him, and he struggles to provide for his teenage daughter, Milly. Opportunity knocks when he is approached by Hawthorne, a recruiter from the British Secret Service, who offers him a lucrative position as a spy. Desperate for money and completely untrained in espionage, Wormold accepts the offer but soon realizes he has no information to report. To continue receiving his salary, he fabricates elaborate reports on fictional agents and imagined plots, sending them back to London.

Wormold's fictitious intelligence garners unexpected attention, and his reports prompt the British government to send reinforcements in the form of secret agents. As international intrigue heightens, Wormold finds himself ensnared in a web of real espionage, confronting both Cuban authorities and enemy spies. The closer the fabricated plots come to being discovered, the more dangerous Wormold's situation grows. Through a series of increasingly perilous encounters and narrow escapes, he grapples with the consequences of his deception. The stakes continue to rise as his double life becomes unsustainable.