Dr. Paul Hoefler, a prominent explorer, organizes an exhibition to track down a secret cannibalistic society in Africa known as the "Leopard Men." The members of this society wrap themselves in leopard skins and conceal their hands in gloves fitted with steel knives shaped like the claws of a leopard, which they use to tear out the throats of their victims. As an initiation rite, the mark of the beast is burned into the body of the initiate, and if he cries out, he is put to death. When this group of assassins comes to the attention of the British government, Hoefler organizes an expedition to photograph the rites of the Leopard Men in hopes of bringing them to trial. After boarding the Kenya and Uganda Railroad, Hoefler enters the interior of Africa. In Nairobi, he meets his faithful servant Ali, who accompanies Hoefler as he treks into Central Africa, the source of the Nile. At the Nile, they board a river steamer headed for the Congo, where a native chief tells Hoefler tha