Ridiculed over his theory that germs cause disease, Louis Pasteur continues his work and eventually finds a cure for anthrax. Despite the warnings of his fiercest critic, Dr. Charbonnet (Fritz Leiber), Pasteur risks imprisonment when he proceeds with the radical idea that injecting humans with the rabies virus will lead to immunization. His beliefs are soon put to the test when Charbonnet agrees to attend Pasteur's pregnant daughter (Anita Louise) but will only sterilize his instruments if the scientist agrees to sign a letter stating his hypotheses are worthless.