In 1860 Paris, a distraught man murders his wife's doctor. [Paul Muni] Louis Pasteur has publicly championed the theory that microbes cause disease, urging doctors to wash their hands and sterilize their instruments in boiling water. The tragedy that follows—his wife dying of puerperal fever after childbirth—becomes a turning point, fueling his relentless pursuit of unseen causes and practical solutions in medicine.
France’s medical academy dismisses Pasteur as a crank, with the outspoken Dr. Charbonnet — portrayed by Fritz Leiber — treating his ideas as nothing more than witchcraft. Pasteur openly challenges Charbonnet’s non-sterile methods and warns that such recklessness could prove fatal. The scientist even predicts that a member of the Emperor Napoleon III’s circle who Charbonnet is tending will die of puerperal fever, a claim that fans the flames of controversy and puts Pasteur in direct opposition to the establishment. Walter Kingsford embodies the Emperor in the story, underscoring the political pressure that shadows scientific progress. Disillusioned by the hostility in Paris, Pasteur withdraws to Arbois, seeking a calmer ground on which to test his ideas.
The narrative shifts to the 1870s, as a new French government grapples with economic recovery after the Franco-Prussian War. They discover that sheep die of anthrax except near Arbois, a clue that points toward Pasteur’s work. The government buys land there and invites sheep farmers to test his approach, while Pasteur remains wary, insisting that the soil itself may be contaminated with anthrax spores. He proposes a bold experiment: vaccinate 25 of the newly arrived sheep and compare them to another 25 that receive blood from an infected sheep. The scene draws in key supporters, including Dr. Lister, a pioneer of antiseptic surgery in England, who attends and witnesses Pasteur’s method. On the personal front, Jean Martel, a young doctor who once assisted Charbonnet but now follows Pasteur, becomes engaged to Pasteur’s daughter, Annette. Donald Woods brings Martel to life, while Anita Louise plays Annette, welding professional ambition to intimate family ties.
Trouble erupts when a rabid dog runs through town and bites a man. A woman’s desperate attempt at cure by witchcraft underscores the stubborn limits of contemporary medicine, and Pasteur laments that doctors may have lost even their best chances of success. Returning to Paris, he makes rabies his next challenge. He experiments by exposing animals to the disease and attempting to shield others with vaccines, but he cannot yet detect any microbial agents—viruses had not been discovered—and the initial vaccine strategy with rabies falters. Dr. Charbonnet returns to gloat over Pasteur’s failures, injecting himself with rabies and seemingly escaping the disease. Pasteur is puzzled until his wife, Marie, suggests that the older sample may have weakened with age, prompting a crucial turn in his thinking. Josephine Hutchinson embodies Marie Pasteur, offering emotional balance to the scientist’s clinical fervor. This hiccup becomes a catalyst, as Pasteur embarks on a more nuanced path, administering progressively stronger injections to dogs and refining the approach.
A desperate mother arrives with her rabies-bitten child—her plea compels Pasteur to act, despite the looming risk of legal and professional jeopardy for practicing medicine without a license. The boy’s case becomes a defining moment, and Pasteur accepts the moral duty to try an untested treatment. At this juncture, Dr. Zaranoff arrives from Russia with a group of peasants exposed to rabies, volunteering for Pasteur’s protocol. Akim Tamiroff embodies Zaranoff, signaling an international dimension to Pasteur’s work. Meanwhile, Annette goes into labor with Martel’s child, and the scheduled birth places additional strain on an already fragile situation. When an available physician cannot attend, Pasteur seeks Charbonnet’s help once more, and Charbonnet agrees to a tense compromise: if Pasteur retracts his rabies work within a month, Charbonnet will concede and withdraw his opposition. The birth proceeds with success, but Pasteur suffers a mild stroke in the process.
Later, news arrives that Pasteur has permission to treat the Russians who remain alive after exposure. He attends them in hospital for the first injections, moving from a wheelchair to a cane as the days progress. The breakthrough vindicates his method, and even Charbonnet yields, tearing up his retraction and requesting the shots himself. In the aftermath, Pasteur learns that the renowned Dr. Lister may denounce him at the medical academy, but the anticipated confrontation dissolves into celebration: Lister praises the work, Dr. Zaranoff presents his Russian medal, and the medical community ultimately honors Pasteur for a visionary line of research that bridged theory and practice, transforming the future of medicine.
