Dr. Gottlieb has already heard of the plague in St. Hubert, for Ross McGurk's worldwide means of transportation and communication keeps the Institute better informed than are the residents of Blackwater itself. It is rumored that Arrowsmith of McGurk may have something to eradicate the plague. Martin is summoned and asked if he will go to St. Hubert and experiment with the phage on half the patients and keep the others as controls. Martin agrees. Sondelius is to go along for publicity and to see that the Institute receives proper credit, though he is not to participate in the experimentation. Martin is to find if possible a district comparatively untouched by the plague and should there have test cases, one half injected with phage and the other half untreated. In the badly afflicted districts, he may give the phage to everyone. If results are favorable, that part of the experiment will be secondary proof. In the meantime, the Surgeon General, Dr. Inchcape Jones, cables that no help is needed. Sondelius considers himself the official rat-killer and is enthusiastic about the job.
When Martin breaks the news of his departure to Leora, she insists on going along since she has no life of her own outside of her husband and his work. He will need her help, she argues, for she has helped him before in the laboratory, and in addition he may become ill. She wins her point.
Gottlieb warns Martin not to let a kind heart spoil his experiment in St. Hubert, for it is necessary to think of generations to come as well as of men now dying. Terry Wickett admonishes Martin to keep his phage notes complete and up-to-date, in ink.
Before the McGurk Commission sails, the island of St. Hubert is under quarantine. Dr. Inchcape Jones has declared that people may come in but that none may go out. Martin buys a supply of tropical-weight clothing to take along, in addition to his syringes and his ampules of phage.
Martin, Leora, and Sondelius sail one winter morning on the steamer St. Buryan, of the McGurk Line. Only Terry Wickett sees them off. It is the first sea voyage for the Arrowsmiths, though Sondelius is a globetrotter. Gottlieb arrives at the dock too late to find them at the rail. He is disappointed.
Martin and Leora are seasick off Cape Hatteras but enthusiastic about their voyage. Leora has brought a West Indian guide book and reads it avidly. Sondelius is all over the ship, striking up acquaintance with all kinds of people. A Miss Gwilliam, from New Jersey, makes him conscious of his lack of culture and refinement, but not for long. Martin becomes conscious of how much he has neglected Leora and promises to do better. So great is her absorption in him that she has never felt neglected and makes light of his attempts to apologize. He promises her that on their return from St. Hubert, they will take the coveted trip abroad so that she may see England and Italy, and most of all, France. She is ecstatic over the prospect of "going places." In their close little stateroom, she tries to make him as comfortable as possible and adores him more than ever.
The representatives of McGurk Institute are drawn into the narrative again and are now on the way to the West Indies, Martin of course being in the lead. To Ross McGurk, the expedition furnishes an opportunity for the Institute to acquire worldwide fame; to Gottlieb, Martin, and Sondelius, it is an attempt to acquire further scientific knowledge, at the same time saving lives. The whole-souled absorption of Leora in her husband is again brought into the narrative, this time leading her into grave danger with him. She admits being lazy, useless, and ignorant except for the protection and love she gives him. Sondelius, once Martin's leader, is now cast in a subordinate though important role. He has qualities which Martin lacks, and the two complement each other. The episode builds up to its highest point, the actual fight against the plague.