After two fallow years at McGurk, Arrowsmith stumbles onto an antibiotic serum that he does not understand (and is unsure how he produced), yet has demonstrated the ability to kill at least one type of germ. He shortly is able to replicate it, and in order to study its efficacy on other microbes is sent to the West Indies, where a virulent outbreak of bubonic plague has arisen. He is coincidentally teamed with a popular Swedish lecturer on "Heroes of Health" he once met while still in South Dakota, Dr. Gustav Sondelius, who is extremely enthusiastic over both the team and the serum's prospects to help cure the disease. Leora accompanies her husband, despite his fear for her safety.