Plot
In 1860 Paris, a distraught man murders his wife's doctor. Chemist Louis Pasteur (Paul Muni) has publicized the theory that microbes cause diseases. Therefore, doctors should avoid spreading by washing their hands and sterilizing their instruments in boiling water. The doctor did not do this, and the wife died of puerperal fever after giving birth.
France's medical academy dismisses Pasteur—notably his most vocal critic, Dr. Charbonnet (Fritz Leiber Sr.)—as a crank whose recommendations are tantamount to witchcraft. Pasteur frankly calls attention to the risks of Charbonnet's non-sterile methods and correctly predicts that a member of Napoleon III's royal family who Charbonnet is attending will die of puerperal fever. Pasteur is considered dangerous because his ideas have led to murder. When the Emperor comes down against him, Pasteur leaves Paris and moves to Arbois.
In the 1870s, when the new French government tries to restore the economy after the Franco-Prussian War, they learn that many sheep are dying of anthrax, except around Arbois. They send representatives who realize that, after working with a small group of loyal researchers, Pasteur developed an anthrax vaccine.
The medical academy still opposes him and says Arbois must be free of anthrax, so the government buys land there and invites sheep farmers to use it. Pasteur objects strongly, saying the soil is full of anthrax spores, and eventually proposes an experiment. He will vaccinate 25 of the newly arrived sheep; then they and a control group of 25 others will be injected with blood from a sheep with anthrax.
Joseph Lister (Halliwell Hobbes), the pioneer of antiseptic surgery in England, is interested enough to attend. He witnesses Pasteur's success as all the vaccinated sheep remain healthy after the other 25 die. At this point, Jean Martel (Donald Woods), a young doctor who was formerly Charbonnet's assistant but now follows Pasteur, becomes engaged to Pasteur's daughter Annette (Anita Louise).
The celebrations are short-lived, as a rabid dog runs through the town and bites a man. As a woman attempts to cure him by witchcraft, Pasteur laments that doctors would have no more chance of success. Moving back to Paris, he makes rabies his next project. He spreads the disease from one animal to another by injection but cannot detect any microbe being transferred (viruses had not been discovered), and the method he used to create the anthrax vaccine fails.
Charbonnet visits the lab to gloat over Pasteur's failure. He is so confident Pasteur is a quack that he injects himself with rabies—and is triumphant, as he does not get the disease. Pasteur is puzzled until his wife Marie (Josephine Hutchinson) suggests the sample may have weakened with age. This sets him on the right path, giving dogs progressively stronger injections.
But before his experiments conclude, a frantic mother begs him to try his untested treatment on her son (Dickie Moore), who a rabid dog has bitten. Despite fearing imprisonment or even execution for Practicing without a license to provide medical treatment, Pasteur decides he must try to save the child. During the attempt, Dr. Zaranoff (Akim Tamiroff) arrives from Russia with a group of peasants exposed to rabies. They have volunteered to receive Pasteur's treatment.
Annette goes into labor with Martel's child. The doctor to attend to her is unavailable, and the boy urgently needs Martel. Pasteur searches for another doctor, but he can only find Charbonnet. He begs Charbonnet to wash his hands and sterilize his instruments; Charbonnet finally agrees that if Charbonnet lives another month, Pasteur will retract and denounce all his work on rabies. Both men are honorable enough to respect the agreement. The birth goes well, but Pasteur suffers a mild stroke.
Days later, word comes that Pasteur has permission to treat the still-alive Russians. He attends them in hospital for the first injections using a wheelchair and later a cane. The experiment is a success, and now even Charbonnet concedes he was wrong, tearing up Pasteur's retraction and asking for the shots himself.
Afterwards, Pasteur hears that Lister will denounce him at the medical academy. He angrily attends, but it is actually a surprise. Lister praises him, Zaranoff presents him with a Russian medal, and the once-skeptical doctors honor him.
