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Off the coast of Scotland, British Naval secret service agent Philip Calvert sneaks onboard the British ship Nantsville to make contact with two undercover agents only to find the men have been murdered. Days earlier, British intelligence head Sir Arthur Arnold-Jones reluctantly assigned Calvert and his intelligence partner, Roy Hunslett, to investigate a series of raids on British ships transporting gold bullion. Under pressure from the Admiralty, the government and insurance company Lloyds of London's, represented by Lord Charnley, Sir Arthur, although uneasy about Calvert's reputation as a maverick, agreed with Calvert's recommendation to place two agents wired with electronic transmitters on the next gold transport ship in order to track them. Now, upon finding the agents murdered, Calvert attempts to leave the Nantsville but is attacked by several men whom he succeeds in fighting off. Calvert rejoins Hunslett on their boat, the FPE Crest , where they are posing as marine biologists on a government-funded expedition. Calvert's report to Sir Arthur is interrupted by the arrival of Scottish police sergeant MacDonald with several customs agents who inspect the Crest briefly. After their departure, Hunslett and Calvert discover that the radio has been destroyed, and Calvert suspects the customs agents were the men who attacked him on the Nantsville . The next morning, Calvert goes ashore to the local village of Torbay, where he learns that telephone service has been disrupted and several villagers have recently disappeared in odd occurrences. While walking near the coast, Calvert meets MacDonald who, when questioned by Calvert, says several local boats have had their radios damaged, including the Shangri-La owned by wealthy philanthropist Sir Arthur Skouras, a generous benefactor to the region. Later, Calvert is beset by two men from the Nantsville , who beat him up and warn him to stay away from the village. Upon rejoining Hunslett on their boat, Calvert receives a message from the nearby anchored Shangri-La inviting them for cocktails that evening. Onboard the lavish yacht, Calvert and Hunslett meet Skouras, his business associates Lavorski and Macullum, and his much younger second wife, who insists on being called Charlotte. Later that night, Calvert phones Sir Arthur, who is indignant over Calvert's request to run a check on Skouras, a member of Sir Arthur's private club. Distressed by Calvert's suspicions, Sir Arthur demands they meet and arranges to send a helicopter to bring Calvert to London the next day. The following morning when the helicopter arrives, Calvert pulls rank on the pilot, Lt. Williams, ordering him to send a false distress signal for the boat, which will then allow Calvert to use the Naval rescue helicopter to fly over the island unquestioned. While Hunslett takes the Crest out to sea, unaware that he has been followed by a small motorboat, Calvert and Williams fly over the island. Twice, Calvert orders Williams to land, once to question shark-fisherman Tim Hutchinson about any unfamiliar ships or activity in the area and once at the clifftop castle Dubh Sgeir. There, Calvert is met by young Sue Kirkside, daughter of Lord Kirkside who reveals that Sue's fiancé and brother have gone missing recently after a flying accident. Later at dusk, Williams returns Calvert to shore, only to have the helicopter fired upon by two men. Calvert manages to crawl back onboard the helicopter, but Williams is badly wounded and the damaged craft soon crashes just off shore. Unable to save Williams, Calvert uses the oxygen mask to remain submerged while the two men explore the area from their motorboat. Exhausted and shaken, Calvert returns to the Crest to find it abandoned, except for a prowler who turns out to be Sir Arthur. The intelligence head explains that his investigation of Skouras revealed that four of the bullion transport ships were robbed within 48 hours of one another and that after a recent financial crisis in the Skouras firm, Lavorski joined the company. Sir Arthur also acknowledges that Skouras' hasty remarriage seems odd because he was deeply attached to his first wife. Considering Hunslett's disappearance as well as those from Torbay, and the attack on the helicopter, Calvert concludes that the Crest will likely be the next target. The following morning, Calvert and Sir Arthur intercept two men sneaking aboard the Crest , then go into Torbay where Calvert questions MacDonald, who admits his sons were taken hostage to enforce his cooperation with the phony customs agents. As Calvert and Sir Arthur row back out to the yacht, they are startled to find Charlotte swimming toward them. Onboard the Crest , Charlotte confesses that she came to warn Calvert that MacDonald reported Calvert's questioning to Skouras, whom she has grown to despise. Calvert decides to sail to the farthest end of the island but upon pulling up the anchor, finds Hunslett's body chained onto it. Calvert then tells Sir Arthur he suspects that the isolated location of Dubh Sgeir castle makes it an ideal place for Skouras to hide the stolen gold. Noting that he and Williams were only attacked after visiting the castle, Calvert insists that they investigate further. On the way to the castle, the Crest is pursued by a motorboat, which Calvert rams, after which he shoots the three gun-wielding men aboard. Calvert then goes ashore to enlist the aid of Hutchinson, who agrees to take Calvert and the others to the castle in his fishing boat, which will not attract attention. Near Dubh Sgeir, Calvert dives and spots a sunken ship on which men in diving outfits are loading gold ingots. When the divers spot Calvert, he is forced to attack and kill them before returning topside. Back onboard Hutchinson's boat, Charlotte flirts with Calvert. That night, Calvert sneaks into the castle where, after killing a guard, he locates Sue, who confirms that the missing islanders, ship crew and the first Mrs. Skouras are all being held in the dungeons. After freeing the prisoners, Calvert goes to the boathouse below the castle, where he sees Lavorski directing the loading of gold onboard the Shangri-La 's launch, which will depart at midnight, or eight bells. Worried by Calvert's long absence, Sir Arthur and Hutchinson ram the doors of the boathouse, but Lavorki's men are armed and waiting for them due to Charlotte radioing ahead information about Calvert's activities. Calvert, who has overhead Charlotte's broadcast and has realized she is Lavorski's wife, causes a diversion allowing Hutchinson and his men to engage Lavorski's men in a fierce gun battle that they eventually win. Calvert then locates Skouras, who declares that insurance man Lord Charnley has been behind the gold heists, and that his wife was kidnapped to compel him to provide cover for the robberies. Hutchinson kills the fleeing Charnley with a shark harpoon. Relieved that Skouras is innocent, Sir Arthur arrests Lavorski's surviving men while Calvert allows Charlotte to escape with a gold ingot.