On a quiet stretch of the English countryside, two schoolboys are exploring an abandoned military base with a model plane when they are intercepted by two men who pretend to be military personnel, scold them for trespassing, and drag them away into a dangerous, unseen plot. The scene sets a tension-filled tone that threads through the rest of the story as the boys’ disappearance becomes a catalyst for a far larger conspiracy.
In London, Major Tarrant [Michael Caine] is an undercover British intelligence officer who is attempting to infiltrate a ruthless arms-smuggling gang supplying weapons to terrorists in Northern Ireland. He makes an initial approach with Celia Burrows [Delphine Seyrig], a keen and dangerous member of the organization, and he documents the operation for his higher-ups before returning to a sprawling country house where Sir Edward Julyan [ Joseph O'Conor] oversees MI6, and Cedric Harper [ Donald Pleasence] commands the surveillance and planning. While there, a call from Tarrant’s wife reveals that their son David has been kidnapped, a personal crisis that the officer tries to compartmentalize as he continues his work, revealing how family pressures can intersect with professional risk in the world of covert operations. The tension between personal loyalty and official duty becomes a secret undercurrent that drives the narrative forward.
As the case unfolds, Harper grows suspicious, convinced that Tarrant has orchestrated the kidnapping or is at least compromised by someone inside the organization. When a mysterious figure named Drabble demands a ransom of uncut diamonds—an exact sum Harper had recently procured for a separate operation—Harper deduces that Drabble must have insiders inside British intelligence. He orders a tightening of the net, while Tarrant, who must keep the mission alive, assigns the arms-smuggling case to another officer and navigates a web of deceit that blurs the line between friend and foe. The Drabble gang even plants incriminating evidence in Tarrant’s flat to implicate Celia Burrows, amplifying the suspicion around his loyalties and casting a widening shadow over the operation.
Tarrant makes a daring move, impersonating Harper over a secure line and arranging to have the diamonds delivered to Paris, hoping to flush out the traitor. He travels to the French capital, where Celia Burrows accompanies him to a dangerous rendezvous site, only to discover that Drabble has murdered her and left Tarrant unconscious beside the victim. Arrested by the French authorities, Tarrant is handed over to British intelligence, and a dramatic rescue attempt by Drabble’s gang ensues, turning the tables on Harper and forcing Tarrant to escape once more. This sequence deepens the mystery: someone within the British establishment is feeding Drabble information, and the survival of Tarrant depends on uncovering that betrayal.
Back in England, Tarrant realizes that the danger is not just the criminals but the possibility of a high-level insider who wants to silence him for good. He adopts a dangerous ruse, pretending to be Drabble and luring senior officers to a rendezvous at the Clayton Windmills near Brighton. The tense confrontation culminates in a dramatic showdown with Sir Edward Julyan, who is forced under duress to admit that he orchestrated the whole scheme to obtain a lucrative retirement, trading his integrity for money and comfort. Rather than negotiate with a traitor, Tarrant demands to know the whereabouts of his son, who is reportedly being held at a black windmill by Drabble. With a cool resolve, Tarrant enters the windmill, eliminates Drabble and his henchman, and rescues David.
The film closes on a note of relief and resilience as father and son walk away together along a rural road, the haunting refrain of a song—Underneath the spreading chestnut tree—reminding viewers of a shared memory beginning and ending the story. The final image leaves a sense of closure tempered by the ever-present vigilance that accompanies those who walk the line between duty and danger, and it underscores the personal stakes that ultimately drive a man to risk so much for family.
