After a drunken binge on the San Pablo waterfront, longshoreman Bobo fears he may have killed a man. In his uncertainty, he takes a job on an isolated bait barge. That night, he rescues lovely Anna from a watery suicide attempt and installs her on the barge. But Tiny, Bobo's longtime pal and parasite, hopes to drive Anna away before domestic bliss tears Bobo away from him; the still unsolved murder may be just the wedge Tiny needs. There's fog on the water and evil brewing...

French-born dock worker Bobo travels from town to town, performing odd jobs and getting drunk at every opportunity. One evening, he comes to San Pablo, California, to meet his Irish friend Tiny, who finds jobs for him. Tiny tells him that there is a job waiting for him elsewhere, and Bobo spends the rest of the night drinking, fighting and flirting with a saloon girl named Mildred. The next morning, Bobo awakens on a bait barge belonging to "Henry" Hirota and his son Takeo. When Henry arrives, he explains to Bobo that they met while Bobo was drunk, and that Bobo agreed to work for him. After policeman George comes by, Takeo explains that Pop Kelly, a grumpy old sailor whom Bobo also met, was choked to death during the night. Bobo is horrified, for he fears that he may be the culprit, and finds Tiny, who wants to go to San Francisco. Bobo agrees, then spends the afternoon with Nutsy, a philosophical night watchman. Bobo and Nutsy are walking on the beach when they see a woman trying to drown herself. Bobo rescues her, then protects her from a policeman trying to arrest her for attempting suicide. The woman, a waitress named Anna, is bitter about Bobo's interference, but Bobo takes her to the barge anyway. Anna, who has led a hard life, cheers up the next day and cooks breakfast for Bobo. He also meets Dr. Frank Brothers, a doctor whose boat has broken down during a cruise with his mistress. After fixing Brothers' boat, Bobo is confronted by Tiny, and Anna overhears Tiny berating Bobo for his heroics. Fearing that she is holding Bobo back, Anna leaves, but returns after Bobo decides to stay in San Pablo and make a home with her. With the help of some paint and curtains, the couple decorate the barge and decide to marry. To avoid the temptation of becoming too intimate before the marriage, Bobo goes with Henry and Takeo in search of bait while Anna tends the barge. Tiny drops by to see Anna and implies that Bobo killed Pop, and that if Anna does not let Bobo go, he will tell the police. When Bobo returns, Anna questions him about his past and Tiny. Bobo admits that his strength can be a liability, for when he was young, he almost choked a jeering cousin to death, and two years previously, he killed a man in self-defense during a fight. Tiny helped Bobo escape and has been living off him ever since. The next day, Nutsy acts as Bobo's best man, and Reverend Price marries Bobo and Anna. Their celebration is cut short by Brothers, whose boat has broken again. While Bobo is repairing it, Anna opens his wedding gift to her, which is a tawdry dress that he bought from Mildred. Anna is dismayed, but Nutsy advises her that a smart wife will not be too modest at home and cause her husband to look elsewhere for excitement. Bobo is still gone when Nutsy leaves, and Anna is upset by the arrival of Tiny, who is more drunk than usual. She offers to keep paying him money each week, but repulses his attempts to kiss her. Tiny keeps advancing, warning her that she cannot tell Bobo or else he will inform the police that Bobo killed Pop. As he talks, Anna realizes that it was Tiny, not Bobo, who killed Pop, and her accusations enrage him. When Bobo and Brothers return, they find the badly beaten Anna and rush her to the hospital. Suspecting Tiny, Bobo chases him out onto the water break, and the terrified Tiny escapes into the water, even though he cannot swim. Bobo tries to help him, but Tiny is washed out to sea by the waves. Bobo then returns to the hospital, where Nutsy informs him that Brothers has sent for a specialist and that Anna will live. Later, Brothers brings Anna to the barge, and Bobo carries her over the threshold.