Gregory Mason, part-owner of a deep-sea salvaging company in Manila, is mistakenly arrested for the murder of his partner, Andy Callahan. Manila Police Lt. Zuenon suspects a love triangle, as the fortune-hunting Mason shares a past with Andy's wife Stella, and possibly a financial motive, as Andy had bragged about hidden treasure just before being killed. As the police interrogation is turning ugly, the oily Steve Ranier shows up claiming to be a private detective hired by Andy. Although Ranier does not identify the murderer, he clears Mason and afterward, introduces him to Benedict, a tropical fish collector with whom, he says, Andy was dealing. Benedict has learned that a man named Ortega carried a box containing $1,000,000 in diamonds out of Manila when he was evacuated by the PT boat captained by Mason during World War II. Mason remembers the man, who died with the sinking of the ship, which only Mason and Andy, who was his executive officer, survived. Benedict offers to split the treasure with Mason and Stella if Mason can locate the shipwreck, but Mason refuses, as he is now secretly making plans to find the treasure alone. Later, Mason is approached by another Ortega, the brother of the man on the boat, who asks for his assistance, but Mason believes he is Andy's killer and rudely sends him away. The next day, Mason finds that Stella, who left him for Andy when he refused to settle down, wants to return to the United States. Mason offers to marry her after he gets the treasure, but believing that he has not changed, she leaves for the airport. That evening, Mason's boat is destroyed in a mysterious fire which kills Perol, the young brother of Mason's faithful employee, Manuelo. After Perol's funeral, Benedict offers Mason a boat, the Mara Maru , to search for the treasure. Mason agrees, but insists that only he have access to the charted course and that only he, Manuelo, Benedict, Benedict's servant Fortuno, and Ranier will take the trip. After they set sail, Mason learns that Stella, too, is on board. Suspicious of Benedict's plans, Mason dives at a spot, secretly knowing it is not the site of the shipwreck, and while he is down, Ranier destroys the radio. Although Mason does not know who broke the radio, he suspects that a double-cross is planned. Stella, who is being wooed by Benedict, believes Benedict is scheming to murder Mason. After telling Mason that she still loves him, she begs him to turn back, but Mason stubbornly persists in his treasure hunt. When they arrive at the real site of the sunken ship, he breaks the compass, then dives, knowing a storm is brewing. He returns with the box, which contains a ceramic cross concealing a second, jewel-encrusted one. Benedict wants to kill Mason, but realizes that without the compass, they need him to get home. When asked by Mason where he stands, Ranier confesses that he is on the side of whoever wins. With the storm disguising his actions, Mason crashes the boat on the shore, and he, Manuelo and Stella escape with the cross. Recognizing that it belongs to the cathedral in Manila, Manuelo wants the cross returned there and Stella agrees. However, Mason is not willing to part with a fortune and during an emotional exchange of words, hits Manuelo, who runs away with the cross. Although Mason is sorry about his outburst, Stella suggests that Mason would kill for money. Manuelo proceeds in the dark to the cathedral, but the next day, Mason and Stella cannot find him. However, Mason sees Ortega enter the cathedral and, leaving Stella outside, follows him to a room in the underground catacombs. When confronted, Ortega explains to Mason that he had asked Andy to help him retrieve the cross, with which his brother was entrusted during the war, but Benedict killed Andy. Wanting the cross for himself, Mason gets rough until Ortega hands it over, begging that he return it to the cathedral and reiterating Stella's words that murder is in his heart. Unmoved by Ortega's plea, Mason leaves the room, but is shot at by Benedict and Fortuno. With the villains in pursuit, Ortega leads Mason to a vestibule connected to the street and to the altar, but then is wounded by a bullet. The police arrive, followed by Stella, who alerted them, and Manuelo. Benedict and Fortuno are arrested and Ortega dies after a last appeal to Mason. With some hesitation, Mason gives Manuelo the cross to return to the altar and Stella returns to him.