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At bar in Manila, the Philippines, Stuart Godfrey meets with divers Dave Courtney, Jack Mautner and Dean Pike to offer them a job retrieving a "valuable object" that he lost when the ship on which he was sailing struck a reef and sank. Although Stuart refuses to divulge the nature of the object, he offers a $1,000 bonus to the diver who finds it. Also joining the treasure hunt is Joanne, whom Stuart has blackmailed into posing as his wife. The attractive Joanne had unwisely stolen a gun, and Stuart threatened to turn her into the police unless she pretended to be his wife to "butter up the authorities." Dean, Dave and Jack have all served in the Navy together, and when they report for duty on board Stuart's yacht, they are introduced to several other divers whom Stuart has recruited to join the hunt. Once the boat casts off, Stuart directs the captain to drop anchor on a deserted island that borders the sunken wreck. He then explains that they are searching for an emerald, and that they have three weeks to find it, after which a Naval salvage operation will take charge of the area. As the divers ready to descend, Stuart becomes visibly nervous when one mentions that a skeleton could exist several years under water before decomposing. Later, in one of the wreck's cabins, Dean is shocked to find a woman's skeleton with a belt wrapped around her neck. The belt buckle bears the initials "E. G." After covering up the skeleton and the belt, Dean surfaces. Wondering what E. G. stands for, Dean sneaks into Stuart's tent and begins to search it. Stealing in behind Dean, Stuart lashes him in the neck with a whip and accuses him of thievery. Later, as Joanne tends to Dean's wound, Dean questions her about Stuart's commitment to their marriage. To prove Stuart's devotion, Joanne shows Dean a bracelet her husband gave her bearing the inscription "from E. G." That night, in a private conversation with Stuart, Dean, referring to the intials E. G. and information he discovered in the tent, calls him Edward, his real name, and accuses him of strangling a woman with his belt, then demands the emerald in exchange for his silence. The next day, Dean surfaces from a dive and hurls a bone from the skeleton at Stuart. Instructed by Stuart to lure Dean away from camp, Joanne invites him for a stroll on the beach. While they are away, Stuart sabotages the valve on Dean's air tank. On Dean's next dive, just as he notices an emerald ring next to the skeleton, his tank malfunctions, causing him to drown. When Dean's body is found bobbing in the water, Stuart posits that Dean was working with a partner who killed him after he found the emerald. Soon after, one of the divers brings the belt to the surface. Relieved that he is now in possession of the murder weapon, Stuart magnanimously offers to free Joanne from her obligation to him, and noticing her interest in Dave, encourages her to go out with him. As Dave and Joanne become romantically involved, Stuart summons Jack and confides that he suspects Joanne is in league with Dean's murderer and has been sneaking out to meet him. Determined to avenge his friend's death, Jack follows Joanne and sees her meet Dave. Later, Stuart shows Dave the damaged valve from Dean's tank and asks him to take charge of insuring the safety of the tanks. When Jack questions Dave's sudden interest in the tanks, Dave explains that Stuart asked him to oversee the valves. Afterward, Stuart sows suspicion in Jack's mind by telling him that Dave volunteered to take charge of the tanks. Now certain that Dave is the murderer, Jack takes a grappling hook with him on his next dive and attacks Dave. In the ensuing fight, Dave slashes Jack's air hose with his knife. On shore, the divers who witnessed the fight defend Dave as acting in self-defense. When Stuart insinuates that Jack attacked Dave in revenge for killing Dean, Joanne, repulsed, no longer can remain silent and denounces Stuart, whom she addresses as "Edward." After shooting Joanne, Stuart jumps into his motor launch and heads for the yacht. As the pursuing divers surround the yacht, Stuart fires his gun at them, and when he runs out of bullets, shoots one with a spear gun. Jumping aboard the yacht, Dave grabs Stuart, and when Stuart tries to lash him with his whip, Dave strangles him with it. Some time later, Dave and Joanne toast each other at the Manila bar in which they met.