A cholera outbreak in a ship's engine room puts pressure on the workers, the engineer, a doctor, and a nurse.
A ship leaves Shanghai in 1932, and Dr. Tony Craig (Chester Morris) tells Captain Mathews (Halliwell Hobbes) he came for a rest. When "sea lawyer" Bilson (Alan Lane) complains about conditions, Gallagher (Alan Hale) knocks him down for chief engineer Crusher McKay (Victor McLaglen). Gallagher provokes a fight with McKay, who goes to Dr. Craig, who learns that McKay set the broken arm of Britches (Barry Fitzgerald). Craig knew nurse Ann Grayson (Wendy Barrie) before but broke a date with her. McKay finds a sick stowaway. In the morning Dr. Craig says he died of cholera. Men stop working, but McKay makes them continue. Craig vaccinates the men and imposes strict rules. Britches says that his number won the Irish Sweepstakes; but he falls sick and dies. McKay finds that Craig tapped into his steam line to boil water. Craig tells McKay that Ann is not coming down to nurse the sick, because she would have to stay; but McKay lets Ann take supplies to Craig.
McKay offers reluctant workers a share of his bonus. Bilson refuses to carry a sick man and escapes out a porthole. McKay lays down before Ann comes in and acts sick; but he jumps up when he hears of heavy weather. Captain Mathews says he plans to go around the storm. McKay lets his friend Metcalf lie down to rest, and other workers stop until McKay joins them shoveling coal. Ann and Craig dream of settling down and having a baby. Craig tells drinking McKay to rest, because he is sick. Craig and Gallagher put McKay to bed by slugging him, Craig depriving Gallagher of the pleasure. Another man collapses and dies, and others fear that McKay is dead. While passengers on deck dance, men plot. Ann suggests that Craig take another job, giving up her dream. McKay hopes to see Frisco with Ann. Four men with shovels of hot coals tell Craig they are going up. Craig says that McKay will not die. McKay stops the four and makes them get back to work. The ship arrives in port, and a woman passenger complains there was no doctor on deck to treat her cold. McKay argues with Craig, who invites Ann to go to Guatemala with him. McKay tells Ann that he didn't intend marriage.
This drama portrays the hard work and dangers of the workers who keep a steamship going. Nurse Ann loves and admires Dr. Craig for his outstanding work on tropical diseases yet wishes him to settle down in her home town. He is tempted by the idea but finds his work calling him and hopes she will join him.