German scientist Paul Ehrlich makes remarkable advancements in the field of medical diagnostics with his research in cellular staining with dyes. His work results in new knowledge about tuberculosis, but also leads to his contraction of the disease. Ehrlich travels to Egypt, where he investigates diphtheria and earns more accolades. Eventually, his research in treating diseases leads to the development of a 'magic bullet' treatment for syphilis.

• Warner Bros. Pictures
• Edward G. Robinson
• Ruth Gordon
• Otto Kruger
• Donald Crisp
• Maria Ouspenskaya
• Montagu Love
• Sig Ruman
• Donald Meek