The day has come for Martin to do his experiment. It's clear that the plague is coming to the village that he's in—St. Swithin's—so he starts injecting half of the population with the vaccine and leaving out the other half. As you might expect, the plague attacks the non-injected half way more.
All the while, Joyce Lanyon cooks Martin's meals for him and acts like a wife, although she says she just wants to help out.
On the evenings after Martin has done his work, he and Joyce tend to find private places to hang out. One night, he leans over and kisses her. Later on, he lingers outside her bedroom door and thinks about going in. But he scolds himself and regrets kissing her when he thinks of Leora.
As his experiment goes well, Martin decides to invite Leora to stay with him. When he tries to telephone her, though, he can't get through.
Meanwhile, Leora waits back in another village feeling lonely and bored. While lying in bed, she remembers that Martin told her to give herself another injection of his plague cure, but she decides to put it off until the morning.
The next morning, she totally forgets about the injection and hangs out in Martin's lab. There she finds a half-smoked cigarette and smokes the rest. Little does she know that there are enough plague germs on that cigarette to kill an entire army regiment.
Two nights later, she wakes up with a fever and headache. When the servants realize she has plague, they all run away, leaving her to die alone. It's a slow and agonizing death, and all Leora can think about while she dies is why her husband Martin hasn't tried to contact her. Meanwhile, he's off kissing Joyce Lanyon in another village.
Right until the end, Leora thinks that Martin will come back and help her. But in the end, she slips into a coma and dies.
Back in St. Swithin's, Joyce Lanyon is gently trying to convince Martin to give his plague cure to everybody in the village, since he now knows that it works. But he remains committed to seeing through his experiment to the end, no matter what the cost.
Martin leaves later that day in a car to get Leora. But he finds her dead in their house and loses his mind with guilt and grief. This is all his fault, and he knows it.
He digs a grave and buries Leora, then tells the folks in St. Swithin's he won't be back. He also becomes a total alcoholic.
This is the last straw for him mentally, and he starts giving out his cure to everyone who asks, thus ruining the experiment he's sacrificed everything for.
Little does he know that a couple of other doctors have taken over his experiment in St. Swithin's and are seeing it through to the end.
By the end of it all, Martin's only job is to make as many vials of the plague cure as he can while others dish it out.
One evening, while sitting alone, Martin hears a knock on the door and opens it to Joyce Lanyon. He knows full well that he was kissing Joyce while his wife was dying, and he has little interest in seeing her. She offers him her friendship, but he tells her to go away and she does.
After a whole lot of injecting and rat exterminating, the quarantine on St. Hubert is lifted. Joyce Lanyon leaves on the first steamer back to America. When she leaves, Martin asks if he could come see her in New York sometime, and she agrees.
Martin eventually returns to America a hero, and the McGurk Institute offers him the command of his own division with a hefty salary.
But all he wants to do is go back to his lab and be left alone. He also feels like a total traitor to Gottlieb for giving out the plague cure to whoever asked and ruining the experiment.