On a street corner in Lisbon, Portugal, sailor Hank Smith, a hulking brute of a man, boasts to his shipmates that as the ship's stoker, he powers the engines of their freighter. When the belligerent Hank incites a fight at a local bar, Tony Lazar, the ship's second engineer, intercedes on Hank's behalf with the police, and together, they return to their freighter. Upon reaching the pier, Tony discovers that the captain has agreed to transport a group of stranded refugees to New York. When Tony learns that Helen Parker, an old acquaintance from New York, is among the group, he goes to her hotel to escort her to the docks. At the hotel, meanwhile, Helen informs her old school friend, pampered heiress Mildred Douglas, that they have found safe passage home aboard the freighter. In reply, the ungrateful, self-centered Mildred complains about traveling on a "filthy little boat." Once the voyage is underway, MacDougald, the ship's chief officer, tells Tony to order the engine room crew to stoke the engines full steam ahead so that they can catch up to a convoy. Below decks, as the men feed the fires that power the engines, Hank, covered in grime and sweat, is invigorated by the challenge and brags that he is the flesh and blood of the engines. Meanwhile, above decks, Mildred complains that the tiny cabin she is assigned to share with Helen is abhorrent. When Mildred notices Tony's interest in the heroic Helen, who has helped the desperate refugees obtain passage to freedom in America, she begins to flirt with Tony and soon convinces him to offer her his larger and more comfortable quarters. To flaunt her conquest of Tony, Mildred directs Helen to deliver her lavish wardrobe to Tony's cabin. Becoming bored, Mildred decides to see how "the other half lives" and tricks Tony into escorting her to the engine room. When Helen learns that Mildred has lied to Tony that the captain has given his permission for the excursion, she criticizes her for jeopardizing Tony's career. Mildred descends into the engine room, and, repulsed by the sight of the brutish Hank, humiliates him by labeling him a "hairy ape." Seething with anger, the misogynistic Hank mounts the stairs to confront Mildred. When Lazar shoves Hank back down the stairs, MacDougald summons them both to his quarters and charges them with allowing their petty animosities to sabotage their mission. Later, MacDougald tells Helen that Tony has decided to leave the ship in New York and asks her to convince him to change his mind. Upon docking in New York, Mildred retreats to the luxury of her apartment. When Helen visits Mildred to inform her that she is returning to Portugal, Mildred treats her like a servant. While Helen is there, Tony phones to ask Mildred for a date, and after she rejects him and hangs up the phone, Mildred turns to Helen and coldly declares that Tony was only a brief diversion. Outraged, Helen flees the apartment. Soon after, Hank and several of his seamen friends arrive at Mildred's address. When Mildred refuses to see him, Hank creates a disturbance in the lobby, and the doorman summons the police, who then arrest him. The caged Hank vociferously protests his imprisonment, prompting the officers to douse him with a fire hose. Hank's friends plead for his release, but when Hank is finally freed, he emerges from his cell a broken man. Drawn by a poster for "Goliath the Gorilla," Hank wanders to the zoo and pauses in front of the gorilla's cage, transfixed. Resolving to "smash" Mildred, Hank proceeds to her apartment building. Meanwhile, Tony visits Mildred to declare his love, but Mildred replies that he means nothing to her and cruelly dismisses him. Crushed, Tony walks out, leaving the door ajar, and after lumbering up the back stairs of the building, Hank enters the apartment through the open door. When Mildred sees Hank looming over her, she collapses in fear, and he carries her to the couch and begins to caress her hair. After she regains consciousness, Hank picks her up, shakes her and then tosses her back onto the couch. His confidence restored, Hank contemptuously tosses a coin at Mildred and leaves. At the seamen's bar, Hank finds Tony passed out drunk at one of the tables and carries him back to the ship. Hank then charges down the stairs to the engine room and orders the men to start shoveling.