Inchcape Jones drives Martin to the village of Carib, worse stricken than Blackwater. The rat-fleas have infected the ground squirrels, and Carib has death in every house. The horrors that Martin beholds almost turn him from his determination to experiment as Gottlieb had ordered. Since Inchcape Jones does not understand the need for experimentation, Martin calls on the governor, Colonel Sir Robert Fairlamb. Sir Robert and Lady Fairlamb are at dinner when Martin arrives. At first, Sir Robert listens politely but later loses his temper and tells Martin that he will do all that is possible to prevent "Yankee vivisectionists" from experimenting in his domain.
Thanks to Sondelius, however, Martin is able to present his case before a special board composed of the governor, Inchcape Jones, Sondelius, Oliver Marchand, and a few others, while Leora listens to her husband's address from the back of the room. All except Stokes and Marchand are against Martin and his experimentation, considering him a fanatic.
Into the midst of the discussion plunges Ira Hinkley, missionary of the Sanctification Brotherhood. He brings up the information that both Arrowsmith and Gottlieb were dismissed from Winnemac and labels them lunatics. Cecil Twyford, of St. Swithin's Parish, arrives too late for the special board meeting but offers the help of St. Swithin's in obtaining what the Board refused.
Four days later, Ira Hinkley is dead.
In Carib, Martin has a chance to give the phage to the population as a whole, and the plague begins to decrease. Now he hopes to try test conditions and soon after that to be on the way home with Leora. It is necessary to burn the village in order to rid it of the infection. Sondelius catches the plague and dies, having refused from the first to be inoculated with the serum.
Panic is increasing in Blackwater, for the people now believe that Martin is withholding from them the drug that can save their lives. Sir Robert Fairlamb is a "blundering hero"; Stokes attends his stricken patients, with only three hours sleep at night; Leora helps Martin prepare phage at Penrith Lodge; and Inchcape Jones goes mad and commits suicide after an abortive attempt to escape from the island. With the appointment of Stokes in Jones' place, Martin, as medical officer in complete charge of St. Swithin's Parish, now has his experiment made possible. He leaves Leora at Penrith Lodge with maids and a soldier butler and takes up his residence temporarily in the Twyford home, to be nearer his great experiment.
Frangipani Court, abode of the Twyford family, is occupied by Cecil Twyford himself, his mother, and five sons. His wife has been dead ten years. A guest in the home is Joyce Lanyon, widow of the wealthy Roger Lanyon, of New York. She has come to the Indies to see her plantations and has been trapped by the plague. So striking is her physical resemblance to Martin that he feels she must be his sister, his twin. She offers to help him with nursing or cooking, and he accepts her offer, all the time intending to send for Leora as soon as possible.
Martin's success as a scientist is saddened by the death of Sondelius, an event foreshadowed by his refusal to be inoculated with the phage until the natives have been served. The demented suicide of Inchcape Jones shows how humans may crack up under pressure. New characters are introduced in the chapter: the Twyford family and the widowed Joyce Lanyon, for whom Martin feels a strange attraction, although he is still faithful to Leora. The West Indies episode is now at its height, and the reader looks forward to the success of the experiment and the return of Martin and the others to New York.