A plague has struck the island of St. Hubert—an island colonized by the British in the Southern West Indies. The governor of the island is Sir Robert Fairlamb, who, prior to the spread of the plague, had fired the rat catcher of the island upon Kellett the Red Leg's suggestion. George William Vertigan, Kellet's nemesis, is upset complains of this decision because he claims that the rats carry diseases. The Surgeon General, Inchcape Jones, however, claims that the island is free of disease, and thus the rat-catcher is unnecessary.
Later, when a ship arrives in the town of Blackwater in St. Hubert, carrying the plague, the disease is spread by rats. Dr. Stokes, the parish medical officer of Swithin Parish warns Inchcape about the plague and its probable spread. Inchcape, however, does not head the warnings of Stokes. Soon there are people dead, including George William Vertigan. Still, there is no action on behalf of the Surgeon General for quarantine or any such remedy. Stokes takes matters into his own hands and contacts Dr. Gottlieb, the director of the McGurk Institute about the plague on the island with a plea, explaining that the plague was about to "flare up and consume" the West Indies.
There are rumors about Martin having a cure for Plague, and yet Gottlieb does not move to action when he hears about the outbreak on the island of St. Hubert. It is not until later, when Ross McGurk talks to him and tells that were they to help the island it could bring the institute world fame, that Gottlieb calls Martin into his office. It is not known why Gottlieb decides to act, whether it is out of kindness or because of McGurk's comments relating to fame. Nevertheless, Gottlieb asks Martin to go to the island and conduct an experiment with his phage in order to deduce its real value. Martin is catapulted into instant notoriety—people interview and everyone is all of sudden interested in him. Sondelius is to go with Martin, though his opinions of what is to be done with the phage differ from Martin's. Sondelius believes that the phage should be given to all. Martin believes that the phage should be given to half, so that he may monitor and take notes and come to a conclusion about the phage. In short, Martin wants to run an experiment, and Sondelius wants to cure everyone. In fact, when it comes time to leave for the island, Sondelius refuses to take his dose of the phage, until everyone is allowed to receive it.
The Board of Trustees agrees to Martin's terms although, Inchcape Jones has not yet admitted to there being a breakout of the plague. Leora insists on joining Martin, and so Gottlieb gives her the title of "Secretary and Technical Assistant to the McGurk Plague and Bacteriophage Commission to the Lesser Antilles." Before Martin leaves, Gottlieb tells Martin to make sure not to let his heart get the best of him and to go ahead with the experiment no matter what; Terry tells him to remember to take careful notes.
The day before the McGurk Commission (Martin, Sondelius, and Leora) sets out for St. Hubert, Dr. Inchcape Jones finally issues a quarantine on the island. Once on the ship, Martin becomes seasick, while Sondelius gallivants and meets Miss Gwilliam, a rich New Jersey spinster who is going to the West Indies to "preserve primitive art." Sondelius and Miss Gwilliam do not, to say the least, get along.
Throughout this section, there is constant attention paid to the amount of love and affection that Leora has for Martin and vice versa.