In 1857, Joel Shore [Robert Taylor] returns to New Bedford, Massachusetts, after three years at sea, and learns that his brother Mark [Stewart Granger], captain of the whaling vessel Nathan Ross, was reported missing from his ship months earlier. Ship owner Asa Worthen [Robert Burton] offers the Nathan Ross to Joel, and although he and Mark were not close, Joel accepts and vows to learn more about his brother's fate.
That afternoon, Joel hears a drunken sailor in a pub claim that Mark jumped ship and abandoned his men, and when the sailor's companion confirms the story, Joel strikes him. Later, Joel calls on his captain, Captain Holt [Lewis Stone], and proposes to Holt's granddaughter Priscilla [Ann Blyth]. Joel and Priscilla marry and immediately depart for the South Seas on the Nathan Ross. One day, Joel questions mate Aaron Burnham about his brother's disappearance, and Aaron replies that Mark had been drinking heavily and fighting a severe fever when he left the ship.
After six months at sea, the ship approaches the island of Tubuai. While anchored offshore, Joel comes up from below deck and discovers Mark on board. The two brothers establish an uneasy rapport, and that evening, Mark tells Joel about the night he left the ship — drunk and ravaged by fever, Mark dives overboard and swims to shore, lured by the sound of the native drums. After Mark collapses, he is nursed back to health by a beautiful native woman, whom he marries in an island ceremony.
One night, Mark awakens to the sound of gunfire, and discovers that his wife has been kidnapped by several white men. Mark swims to the sloop and kills the man who is trying to rape his wife, but then collapses. Mark and his wife remain barricaded in the ship's cabin while the other two men, Quint [Kurt Kasznar] and Fetcher [James Whitmore], bide their time on deck. When Mark recovers his strength, he confronts Quint and Fetcher at gunpoint, and agrees to join them in their next pearling expedition.
One day, while examining their haul, Quint comes across a valuable black pearl, and Fetcher murders him for it. Several weeks later, they stop at a small island to pick fruit, and Fetcher murders their two native divers, then tries to kill Mark. Mark chokes Fetcher to death, but as he and his wife are leaving the island, they are attacked by natives. Mark's wife is killed by a native's spear, and Mark accidentally drops the bag containing a fortune in pearls into the lagoon.
Back aboard the Nathan Ross, Mark tells Joel he intends to go back for the pearls. Fearing the hostile natives, Joel refuses to join him, and instructs Mark not to mention the pearls to the men. Mark nonetheless tells one of the mates, and word of the sunken treasure soon spreads throughout the crew. Later, Mark tells Joel that he might have married Priscilla himself, and intimates that he can take her away. Mark plants doubts in Priscilla's mind about Joel's courage.
One night, Mark and some of the mates announce that they will go after the pearls, and Mark demands that Joel turn over his guns. Interpreting Joel's lack of resistance as a sign of cowardice, Priscilla is ashamed of her husband, and as Mark comforts her, they kiss. Joel assembles the men and tells them they will not search for the pearls, proposing instead that he and Mark return to Tubuai and fight a duel.
Mark then seizes control of the ship and has Joel taken prisoner. Joel escapes and, after ordering Mark to end the mutiny, throws all their firearms overboard. The angry crew attacks and Mark — determined not to have his brother's blood on his hands — is forced to fight on Joel's side. Mark is killed during the brawl, impaled by a whaling harpoon.
Later, Joel writes in his captain's log that although Mark instigated the mutiny, he later regretted his actions and defended the ship. With their troubles behind them, Joel and Priscilla reconcile and kiss.
