The Black Windmill

Two schoolboys are playing with a model plane on an abandoned military base in the English countryside. They are approached by two RAF personnel who rebuke them for trespassing and take them to see their commanding officer. It soon becomes apparent that the boys are being kidnapped.

In London, Major Tarrant, an intelligence officer, begins an undercover operation to infiltrate a gang of arms smugglers who sell weapons to terrorists in Northern Ireland. He makes an initial approach with Celia Burrows to gain their trust. He then goes to a large country house to report to MI6 head Sir Edward Julyan and his direct superior Cedric Harper.

While at MI6, Tarrant receives a phone call from his wife reporting that their son David has been taken. A threatening call follows, and Tarrant remains calm, informing his superiors that it is a family problem. He is given permission to leave to handle the situation.

Drabble calls again, revealing that he knows who Tarrant is and what he does. He tells Harper to arrange a ransom of uncut diamonds and set up a meeting in Paris. The next phone call is to be answered by Harper.

Harper concludes that someone within British intelligence is leaking information. He begins a surveillance operation on Tarrant and assigns the arms-smuggling case to another officer. This marks the start of intense internal scrutiny.

Drabble's gang plants incriminating evidence in Tarrant's flat, implying a link to Celia Burrows. Scotland Yard finds the evidence during a search, fueling Harper's suspicions that Tarrant is involved. Tarrant is placed under closer scrutiny as a suspect.

Tarrant secretly impersonates Harper on a secure telephone and arranges to have the diamonds made available. He then takes the diamonds to Paris and evades the tail Harper has placed on him. He is acting to outmaneuver the investigation and reach the kidnapper's rendezvous.

In Paris, Celia Burrows escorts him to a building where his son is allegedly held. Drabble's plan soon reveals itself as Celia is murdered, and Tarrant is left unconscious.

Tarrant is arrested by the French police and handed over to Harper and British intelligence. The Drabble gang stages a rescue, freeing him from custody but attempting to murder him.

After escaping, Tarrant returns to England to uncover the traitor inside British intelligence. He pretends to be Drabble and arranges a rendezvous at the Clayton Windmills near Brighton to draw out the culprit.

Sir Edward Julyan arrives at the windmills to meet Drabble. Under duress he confesses that he arranged the whole scheme after being passed over for promotion. He tries to offer half the diamonds but Tarrant demands to know where his son is.

Tarrant enters the windmill, kills Drabble and his henchman, and rescues his son David. The rescue marks the collapse of the conspiracy and the safety of the boy is ensured.

The film ends with Tarrant carrying David along the road away from the windmill as the familiar song plays. The image mirrors the opening and underscores the film's conclusion of family triumph over treachery.