The famous microbiologist warns doctors about spreading germs and proves that his vaccines for anthrax and rabies are effective.
In 1860 Paris a man shoots a doctor for killing his wife by not washing his hands as advised by Louis Pasteur (Paul Muni) because of microbes. Pasteur says that three of ten women are dying in child-birth because of germs from doctors. Napoleon III invites Pasteur, who accuses Dr. Charbonnet (Fritz Leiber) of killing his patient. The emperor orders Pasteur to confine himself to preserving wine and beer and to retract his pamphlet. Pasteur, his wife Marie (Josephine Hutchinson), and daughter Annette (Anita Louise) leave Paris.
After the 1870 war, French President Adolph Thiers learns that some cattle are immune from anthrax and orders an investigation. In Arbois doctors Radisse and Jean Martel (Donald Woods) find Pasteur using a vaccine. Radisse leaves, and Martel stays. Pasteur shows Martel the anthrax germs in blood. Radisse sends sheep to Arbois; Pasteur and Martel try to stop them. The Academy of Medicine debates, and Pasteur accepts an experiment of injecting sheep with blood of dead sheep. Dr. Lister (Halliwell Hobbes) comes from England. All the sheep not vaccinated die, and all those vaccinated live. Martel and Annette ask Pasteur for permission to marry. Dr. Rosignol (Porter Hall) and Lister congratulate Pasteur.
People run from a rabid dog. In Paris a year later pregnant Annette worries. A letter from Lister says hospitals are becoming safe. Pasteur promises a cure for hydrophobia (rabies). Dr. Zaranoff (Akim Tamiroff) persuades Charbonnet to go see Pasteur, and Charbonnet injects himself with rabies to disprove Pasteur's germ theory. After a month he laughs at Pasteur, who then realizes the germs were weakened by age. Pasteur has Martel inject dogs with old germs. A doctor brings a boy bitten by a rabid dog. Pasteur calls in Rosignol, who advises against trying it. Risking prison, Pasteur injects the boy. The third injection made him ill, but he will give more. Marie tells her husband to rest. Zaranoff arrives with Russian patients for a trial. Pasteur looks for a doctor for Annette and finds Charbonnet. Pasteur begs him to boil his instruments; but he makes Pasteur sign a statement his rabies research is fruitless. Pasteur makes Charbonnet wash. Charbonnet says that Pasteur had a stroke. In a wheelchair Pasteur treats 19 Russians, and Charbonnet rips up his statement and asks for treatment. The Russians thank Pasteur. Several years later Marie gets Pasteur to go to the Academy, where Lister leads a tribute to him. Pasteur speaks to the young doctors and scientists about working for humanity.
Pasteur's story is structured to dramatize his discoveries and makes many wonderful points about how to prevent and cure disease; but perhaps more significantly it shows the skepticism an innovator must overcome with hard work and perseverance in order to make progress for humanity.