French chemist and microbiologist Louis Pasteur (Paul Muni) is on a quest to cure the ailments of the 19th century. But he's thwarted at every turn by skeptical fellow scientists, chief among them Dr. Charbonnet (Fritz Leiber). He draws scorn when he supports the germ theory, advocating that doctors should wash their hands and sterilize their instruments before working on patients. But Pasteur perseveres, and when anthrax becomes a scourge, he holds the key to solving the epidemic.