News of the plague outbreak in St. Hubert reaches the United States, and word eventually gets around that Martin Arrowsmith of the McGurk Institute might have something that can stop the outbreak and save the people of St. Hubert.
Max Gottlieb couldn't care less about these people, since he's only interested in science. But Martin knows that he needs to do something.
Gottlieb eventually agrees to send Martin to test the phage vaccine in St. Hubert. But here's the horrible catch: he wants Martin to use St. Hubert as a science experiment. The only way they can be sure the phage works is to only give it to half the infected people and to let the other half suffer and die. This means hundreds of deaths will be on Martin's hands; but the experiment might prevent hundreds of thousands of deaths in the future.
The one real snag in Martin's big plan is the fact that his wife Leora insists on going with him. Of course he says absolutely not. But she won't budge and he eventually gives in. As you can imagine, having Leora beside him is going to make it more difficult to let a bunch of people die for the sake of testing a vaccine.
Sondelius has decided to come along too, for yucks and, oh yeah, for science.
The day before Martin and his crew sail, St. Hubert declares quarantine: people are allowed to come to the island, but no one's allowed to leave.
While they travel, Martin has a chance to talk with Leora and to apologize about how he has always neglected her to focus on his work.
Leora says she genuinely doesn't mind, but Martin refuses to believe this. Leora tells him to stop being a baby and to simply accept the fact that she doesn't require a whole lot from him. Nonetheless, Martin promises that once they're done in St. Hubert, he's going to take her to France like she's always wanted.