While on their honeymoon, Nan and Bradley Collins, who have known each other for only a week, run into Brad's former girl friend, fashion photographer Christine Norman. Still in love with Brad, Christine bristles with jealousy upon meeting Nan, and later confers with her Communist party cohort, J. T. Arnold, about Nan's background. Christine learns that Brad, the vice-president of San Francisco's Cornwall Shipping Company, has obtained a stevedore job for Nan's younger brother, Don Lowry. Cornwall Shipping is in the midst of a labor dispute, and president J. Francis Cornwall assigns the much-respected Brad, a former dock worker, to act as an intermediary between management and the union. Brad's new authority is endorsed by Jim Travers, the head of the union and Nan's recent ex-boyfriend. Before Brad can act on his assignment, however, he is visited by Vanning, a secretive Communist leader, who reminds Brad about his past as a Communist agitator named Frank Johnson. Annoyed, Brad goes to see Christine and declares that he has long since "graduated" from both her and the "party." Vanning counters Brad's defiance by ordering him to the party's dockside headquarters and threatening to expose him unless he donates two-fifths of his salary to the party. Vanning also forces Brad to witness the brutal drowning of Ralston, a party member suspected of being an FBI informant. Despite Vanning's vicious tactics, Brad announces that he is going to tell Nan and Cornwall the truth about his past. Producing a photostat copy of a party document written by Brad years before, in which Brad assumed responsibility for a killing that occurred during a walkout he orchestrated, Vanning effectively ends Brad's resistance. Faced with the prospect of a murder conviction, Brad agrees to help Vanning shut down the waterfront for sixty days by sabotaging the labor talks. The rebuffed Christine, meanwhile, pursues Don, much to Nan's dismay, and subtly draws him into her Communist circle. During the doomed labor meetings, Don loudly repeats the "party line," agitating his fellow workers, while Brad counsels management to reject all of labor's compromises. Finally, the negotiations are canceled, and the waterfront is shut down. Concerned about Christine's growing attachment to Don, the coldhearted Vanning demands that she break all ties with him. As Christine is about to leave for an assignment in Seattle, however, Don proposes marriage, and a conflicted Christine pledges her love. Sensing that Brad is in trouble, Nan, meanwhile, presses him to confide in her, but he denies any trouble. When Nan hears that Don has proposed to Christine, however, she confesses to Jim her apprehensions about the photographer, and Jim reveals that Christine is a Communist. Jim then accuses Don of being Christine's "stooge," and although he denies the charge, Don later asks Christine if Jim's allegations are true. While insisting that she truly loves him, Christine admits her Communist involvement to Don, but also implicates Brad in the conspiracy. Vanning overhears Christine and Don's conversation and, worried that Don will expose Brad, arranges for Don to be murdered by hired killers Bailey and Grip Wilson. Just after Christine telephones Nan to warn her about Don, Don is run down outside Nan and Brad's apartment. Although Brad tries to convince Nan that the death was accidental, Nan is suspicious and goes to see Christine. The grief-stricken Christine reveals that Don was indeed murdered, but also shows Nan a copy of Brad's party membership. With information provided by Christine, Nan finds the womanizing Bailey at the shooting gallery he operates and, while posing as an unhappily married rich woman, cajoles Bailey into revealing he is a hired killer. Vanning, meanwhile, forces Christine to write a suicide note and pushes her out of her apartment window, just as Brad arrives looking for Nan. Brad tries to retrace his wife's steps, but is waylaid by Grip, who has been notified by Vanning of Nan's activities. Eventually, however, Brad tracks Nan to the party warehouse, where she is about to be killed by Vanning. Brad bursts in on Vanning, brandishing a gun, but Vanning pulls out his own gun and tries to outdraw Brad. In the ensuing confusion, Brad and Nan escape and hide themselves in the warehouse. While exchanging gunshots with Vanning and his men, Brad apologizes to Nan for his past. Vanning finds Nan and is about to shoot her when Brad charges him. Brad manages to throw a grappling hook into Vanning, causing him to fall to his death, but is shot several times in the process. As a forgiving Nan holds him in her arms, Brad whispers that he met her too late and then dies.