Dr. Kurt Radnor, head of the Tropical Disease Institute in Africa works with Dr. Kabayochi and Dr. Roy Stark to find a cure for sleeping sickness, which is known locally as "nagana."

Roy falls in love with Countess Sandra Lubeska, a notorious seductress, and Kurt comes to get him at her villa.  Roy reluctantly leaves, but Kurt stays on to reprove Sandra and falls in love with her.

Upon returning to his lab the next day, Kurt discovers that Roy has killed himself.  Kurt is deeply affected by Roy's death, and decides to continue his work at the native village of Nagoru, after quelling Sandra's pleas to accompany him.  Kurt and Kabayochi are informed by the native king's son, Nogu, that if the king dies while they are working, Kurt's life will be taken.

With the help of guides, Sandra joins Kurt, for she has sincerely fallen in love with him, but the king falls ill and the natives believe that she is a witch.  Kabayochi, also ill, tries an untested serum on himself, and Sandra steals some and injects the king.  Both the king and Kabayochi die, and Sandra and Kurt are sentenced to death.

While Sandra is tied to a post next to a crocodile-infested river, Kurt is given twelve hours to complete a serum he is sure will be successful.  Kurt infects Nogu with nagana, and uses the serum on himself and Nogu.

The serum works, and Nogu orders Sandra's release, but the witch doctors defy him and kill him.  Kurt burns the laboratory and releases the wild animals he has used for tests, then escapes with Sandra.