Two schoolboys play on the field of an abandoned British Ministry of Defense base and are confronted by men in uniform for trespassing. Inside the barracks, a team of kidnappers knock the boys unconscious. As Ceil Burrows drives away with the boys, her co-conspirator, McKee, uses a remote device to take control of their partners’ Land Rover and guides the vehicle into an explosive crash. Later, intelligence agent Major John Tarrant goes to Ceil’s home, unaware of the kidnapping. He poses as a recent ex-convict and one of her lover’s new recruits. Ceil flirtatiously tells him to return in a week while McKee watches from behind the bookcase. Outside, Tarrant’s partner, Mike McCarthy, urgently escorts him to a meeting with General Purposes Committee director Cedric Harper. At the country manor house of Secret Intelligence Service head Sir Edward Julyan, Tarrant and Harper report their efforts to thwart a sabotage ring that supplies Soviet arms to Northern Ireland. While Harper explains that the ring leaders are demanding a payment of £517,057 in uncut diamonds from undercover agents, Tarrant receives an urgent call from his estranged wife, Alex. She received word that their son, David, is in trouble. At Alex’s home, Tarrant gets a call from a kidnapper named Drabble who claims to have intimate knowledge of the Major. Although Tarrant denies his connection to Harper, Drabble demands to speak to the director. Tarrant finds Harper at a shooting range, testing a briefcase-enclosed gun, and although the director is more concerned about the security breech than the kidnapping, he agrees to take the call. Back at Alex’s house, Scotland Yard sets up a phone tap and Drabble calls, telling Harper that his organization is responsible for the Land Rover explosion. A call from another man who identifies himself as Drabble demands a ransom of £517,057 in uncut diamonds, the exact amount of the secret arms deal. Drabble knows Harper is already in possession of the diamonds and threatens to kill David in twenty-four hours. Before hanging up, Drabble puts David on the line and he screams in terror. Meanwhile, David’s friend resurfaces and Scotland Yard detectives suspect him of masterminding the kidnapping. Back at Julyan’s manor house, Harper draws the gentleman away from a dinner party to report that Drabble was informed by an insider and Tarrant is under surveillance. Julyan asserts that the government will never agree to the ransom but tells Harper to play along. The next day, Ceil and McKee break into Tarrant’s apartment, take a nude photograph of Ceil in his bed and leave incriminating evidence behind. Later, Scotland Yard detectives search Tarrant’s apartment and find photographs of his undercover meeting with Ceil, the nude picture, a receipt for a diamond ring and two passports with false identities. Back at Alex’s house, Tarrant receives a tape from Drabble. Alex accuses her husband of having no feelings, but Tarrant tells her his stoicism might be the only way to save their son. Tarrant returns to his apartment to find his phone bugged and takes precautions to cover the sound of the tape before playing it. Drabble tells him to fly to Paris alone with the diamonds and to ignore the instructions for Harper. However, Drabble gives Harper the same message and tortures David when the director protests. Back at his office, Harper tells Tarrant that he will not hand over the diamonds. Later, after Harper leaves, Tarrant steals the safety deposit key for the diamonds and the director’s briefcase weapon device. Impersonating Harper, Tarrant calls the bank and tells him that he will be collecting the jewels. Before he leaves for Paris, Tarrant tells Alex his plan and they embrace. Harper soon learns about the theft and sends men to trail Tarrant. After a harrowing chase through the London Underground, Tarrant arrives at a Paris café and is met by Ceil. They take a taxi to a building where McKee waits with an armed guard. McKee tells him that David is at an English farm with two windmills and demands the diamonds up front. When Tarrant refuses, McKee threatens to kill him, but Tarrant explains that the briefcase holding the diamonds is connected to explosives. Claiming that David is in the building, McKee leads Tarrant into a wine cellar where he punches the Major, causing the briefcase to discharge. As wine gushes into the room, McKee takes the diamonds and throws them to Ceil. Later, the French police find Tarrant beside Ceil’s strangled body. As Harper escorts Tarrant away from the hospital and accuses him of murder, McKee follows and motions to his men in a nearby truck. The police vehicle is forced off the road and crashes. McKee’s cronies release Tarrant but he is chased by a police motorcyclist. Dodging bullets, he jumps onto a barge. After escaping back to England on a hovercraft ferry, Tarrant calls Alex with a secret message undecipherable to the officer tapping her phone. Alex asks the guards for a lift to her Aunt’s house, then slips out the back to meet up with Tarrant. He speculates that Drabble is a member of the General Purposes Committee and asks Alex to research the location of the two windmills. After hearing back from Alex, Tarrant impersonates Drabble and calls the committee members, starting with Harper, instructing them to come to the two windmills. Sometime later, Julyan arrives at the farm and Tarrant knocks him down, demanding his son. With Tarrant’s hands at his throat, Julyan says David is in the black windmill. As Tarrant escorts Julyan to the windmill, the gentleman confesses he needed the money to keep his wife entertained after his forced retirement and claims David was never in danger. Once inside the windmill, Tarrant overcomes the guard, takes his machine gun and fires at McKee. Backing away, McKee starts a fire between them with his oil lamp. Tarrant charges through the flames but McKee waits for him behind a trap door. Tarrant protects himself with a sack of flour and chases McKee toward the top of the windmill, where he fires through the floorboards. McKee falls to his death. Climbing the stairs, Tarrant finds his son in tears. As he carries David away, Tarrant says that he will bring him back to his mother because they both love her.