In a tense Cold War backdrop, a routine handover at a Secret Service desk kicks off a high-stakes espionage thread. Roger Allsop [John Le Mesurier] passes along some belongings to a clerk, who stuffs them into a drawer marked 007 before turning the identifying card over to reveal the word deceased. His superior, Col. Cunliffe [Robert Morley], weighs the necessity of sending a courier behind the Iron Curtain to retrieve a critical item, setting the plot in motion.
An unemployed writer, Nicholas Whistler [Dirk Bogarde], is recruited to appear for an interview with Cunliffe, supposedly for a trainee-executive role at a glass company. Cunliffe quickly discovers Whistler’s fluency in Czech, and offers him a generous salary and coverage of expenses, signaling that this assignment carries more than a normal job prospect. The mission unfolds with cryptic direction: meet someone who will answer to the coded prompt “Hot Enough For June,” and then depart for Prague on a strictly labeled “business” trip later that day.
Upon arrival, a capable driver and guide, Vlasta [Sylva Koscina], escorts him to a glass factory, where a washroom attendant delivers a crucial clue in response to the code. That evening, Whistler accompanies Vlasta to dinner, and a mutual attraction begins to simmer beneath the surface of the burgeoning danger. Unbeknownst to him, Vlasta is an agent for the Czech secret police, who already know—though Whistler does not—that he is working for British intelligence. The pair grows closer, and Vlasta invites him to spend the night at her unexpectedly luxurious home.
The following day, Whistler returns to the factory and receives a slip of paper from the washroom attendant, who warns that they are both spies. Vlasta arranges a secret rendezvous that night and cautions him to return to England immediately. Instead, Whistler finds himself confronted by Simenova [Leo McKern], the head of the secret police, who offers him a stark choice: sign a confession or face a fatal accident. Whistler creates a distraction and escapes, turning to the one person who might aid him: Vlasta.
Whistler and Vlasta’s uneasy alliance deepens as he confronts the possibility that his pursuers will close in. He heads to Vlasta’s home, only to discover she is preparing to meet her father, Simenova, before departing for work. After Simenova leaves, Whistler challenges Vlasta, who proposes a risky plan to help him reach the British embassy despite a police cordon. To demonstrate his good faith, Whistler burns the crucial slip of paper he is supposed to deliver, ensuring that neither side can claim it. The escape plan almost succeeds, but Simenova happens to be leaving the embassy as Whistler nears, forcing another frantic defection.
Desperation drives Whistler to improvise a final, clever maneuver: he disguises himself and infiltrates the embassy as a milkman, slipping past the guards. Cunliffe informs him that he is to be exchanged for a captured spy, a twist that shifts the balance of risk and reward. At the airport, Whistler is pleasantly surprised to learn that Vlasta has been assigned to the British trade mission in London, and the two reunite with a seat beside each other as the plane takes off toward the capital.
