The plague has only begun to invade St. Swithin's when Martin divides the population into two equal parts, injecting one half with the plague phage and leaving the other half unprotected. The unphaged half has far more victims than the other, and Martin is elated. Cecil Twyford risks his life nursing his servants, and Joyce Lanyon takes over the cooking, bed making, and other duties.
The bond of resemblance as brother and sister is tightened as Martin and Joyce work together, although he struggles against his liking for her. After trying for two hours to reach Leora by telephone, he gives up, assured that within three or four days he can go for her and bring her to St. Swithin's.
Left practically alone in the big house, Leora is lonely and sleepless. She has forgotten to take another injection of the phage as Martin had asked her to do. News comes that Oliver Marchand, the black physician on whom she is dependent for medical care, is dead. The butler leaves to go to his sister, ill of the plague. Leora goes to Martin's laboratory, picks up one of his half-smoked cigarettes, and lights it.
A maid had knocked over a test tube on the cigarette, and it contained enough plague germs to kill a regiment. Two nights later, Leora becomes violently ill and dies alone in her bedroom, calling for Sandy until she falls into the coma preceding death.
Joyce Lanyon, like Sondelius, tries to persuade Martin to give the phage to everyone. He explains Leora to Joyce before going to Penrith Lodge for his wife, only to find her stiff and lifeless. He goes to pieces and begins drinking heavily. He does not care whether he lives or dies and sits talking to Leora and Sondelius, Hinkley, Marchand, Inchcape Jones, and others among the dead. For some time he does not see Joyce Lanyon, but one day she appears, straightens his room, and offers him friendship, which he rejects and goes back to rigid observation of his St. Swithin experiment.
The epidemic subsides, and six months after Martin's coming, the quarantine is lifted. The first steamer leaves, with Joyce Lanyon aboard. Martin has asked to come to see her in New York. All the while, he is mourning for Leora.
Martin becomes a dignitary because of his success with the phage. No one heeds a Scotch doctor who hints that plagues sometimes run their course without phage. A letter from Holabird brings news to Martin that Gottlieb, in poor health, has resigned as Director of McGurk Institute and that Holabird is now Acting Director. Martin is offered a promotion as head of a new department. He feels that he has been a traitor to Gottlieb and all that the old scientist represents. In spite of an elaborate farewell dinner the evening before his sailing, Martin dreads explaining matters to Gottlieb and Terry Wickett.
Death removes two important characters from the novel: Leora and Sondelius. Lewis probably gets rid of them so as to give Martin more freedom of action upon his return to New York. He is eager to get back to his lab and "start all over again." His friendship with Joyce Lanyon foreshadows events to come. The excitement and horror of the West Indian part of the story end with this chapter, and another change of scene is expected. The beauty of the landscape is brought out in sharp contrast to the intensity of human suffering the chapter contains. An example of local color used as contrast follows: From a hilltop they swung down a curving road to a beach where the high surf boomed in limestone caves. It seemed impossible that this joyous shore could be threatened by plague, the slimy creature of dark alleys.