In the early 1960s, a British businessman named Greville Wynne is approached by the UK's MI6 and the CIA to become a courier for crucial intelligence. Wynne, initially an unsuspecting and ordinary figure, is chosen for his unassuming profile to establish contact with Oleg Penkovsky, a high-ranking Soviet official wanting to defect. Penkovsky provides vital information about Soviet missile plans that could drastically alter the course of the Cold War. Wynne accepts the mission, believing it to be a minor role, but soon finds himself embroiled in high-stakes espionage. As he travels between London and Moscow, Wynne's life takes a sharp turn as he balances his normal business activities with his secretive government work.

As increasingly perilous situations unfold, Wynne and Penkovsky's alliance grows stronger, yet their mission becomes ever more dangerous. Tensions escalate as they gather and relay critical information that has global implications. The intelligence sourced by Penkovsky and transported by Wynne is invaluable in providing insights into Soviet plans, especially in the context of rising geopolitical tensions. However, the constant threat of detection by the KGB looms large. As the operation progresses, the bounds of trust and loyalty are tested, leading to an intense and gripping sequence of events that underscores the human cost of espionage during one of history's most tumultuous periods.