On the docks of San Pedro, Tom Merriam, a young third officer on his way to his first assignment, is warned by a blind beggar about boarding the Altair , the ship to which he has been assigned. Ignoring him, Tom mounts the gang plank and passes Finn, a mute sailor who possesses prescient insight. Tom is welcomed aboard by Capt. Will Stone, then is taken aback when the captain criticizes him for killing a moth. Soon after, the body of a missing crew member is found, dead from a heart attack, prompting Finn to muse to himself about the death and agony that will soon pervade the ship. Once the voyage is underway, a freshly painted giant hook breaks loose and swings precariously over the deck. When Tom comments that the hook should be secured, the captain refuses because the paint job would then be marred. Afterward, the captain challenges Tom's remarks regarding the hook, megalomaniacally asserting his divine right over the crew. After a sailor collapses with appendicitis, an onshore doctor radios instructions to operate. Unable to make the incision, the captain freezes and Tom takes over, successfully completing the surgery. Out of respect for the captain, Tom gives Stone credit for performing the operation. Now short two men, Louie, one of the sailors, suggests to the captain that they pull into port to replenish the crew. Soon after, Louie is scrubbing out a chain locker that contains the ship's anchor. Upon completing the job, Louie signals for the heavy chain to be fed back into the locker. At that moment, the captain jams the escape hatch, trapping Louie beneath the crushing weight of the chain. Tom confides his suspicions that the captain is responsible for Louie's death to Bowns, the first officer. When Bowns dismisses Tom's concerns, Tom tells Sparks, the radio operator, who advises him to mind his own business. Upon docking in San Sebastian, Tom files a complaint about the captain to Charlie Roberts, the head of the shipping office. An investigation is held, but the sailors all testify on the captain's behalf, commending his courage during the surgery and noting Tom's poor judgment in the case of the unsecured hook. Defeated, Tom resigns his post on the Altair . Feeling sorry for the young officer, Ellen Roberts, Charlie's daughter and Stone's sweetheart, arranges for him to meet her younger sister, who lives in San Pedro. On board the ship that night, Ellen tells Stone that her divorce is final and she is now free to marry him, and is shocked when Stone responds that he fears he is losing his mind. On shore, meanwhile, Tom comes to the aid of a fellow crewman involved in a street brawl and is knocked unconscious. Unaware that Tom has decided to leave the ship, the sailors carry him back onboard the Altair , which then sails at midnight. Upon regaining consciousness, Tom, fearful that the captain plans to kill him, asks Sparks to send a wire to the shore, but he refuses. Tom discovers that the lock to his cabin door has been removed and when he is startled by strange noises in the night, he sneaks into the captain's cabin to search for a weapon to defend himself. There, he is confronted by Stone, who insanely rants about authority and defies Tom to find allies among the crew. When Tom turns to the crew for help, they all accuse him of mutiny, except for Finn, who senses the depths of the captain's hatred of Tom. Soon after, Roberts telegraphs the ship, inquiring if Tom is aboard. When the captain hands Sparks a negative reply, the radio operator becomes suspicious and shows the message to Tom. Upon leaving Tom's cabin, Sparks is surprised by the captain, who escorts him along the deck. Sparks lets the message slip from his hand, and it is recovered by Finn. The next morning, the captain informs the crew that Sparks has fallen overboard. When Tom accuses the captain of murdering Sparks, the crew thinks that he is insane and sedates him. Desperate, Finn delivers the message to Bowns, and as the first mate and crew debate over what action to take, the captain overhears their conversation, goes berserk and snatches a dagger from his drawer. As the captain is about to plunge the dagger into Tom's heart, Finn springs from the shadows, and in the ensuing struggle, stabs the captain to death. Reinstated as third officer, Tom sails the Altair to San Pedro, where he is met by Ellen's sister.