American insurance investigator Scott Walters is sent to London to protest the risky practices of one of his clients, Perry Henderson of the Henderson House diamond merchants. After Perry's identical cousin Arthur meets briefly with Scott and tells him that Perry is in Africa, Scott tries to get information from the Hendersons' secretary, Marian Taylor, but she rebuffs him. That evening, fueled by his attraction to her and his need to clear up mysteries about Henderson House, Scott calls every "M. Taylor" listed in the phone book until he finds her, then shows up at her apartment. After convincing her to take him sight-seeing the next day, he learns that she is Perry's fiancée. Later, he reads in the newspaper that Perry died at sea while searching for diamonds off the ship Nigeria , and returns immediately to Henderson House. Learning nothing there, he goes to Marian's apartment and finds that she has left the country by plane. Scott suspects that Marian went to Africa, and that Perry is alive and cheating the insurance company, and flies to Johannesburg. There he boards the Henderson-owned ship, Nigeria , and finds Marian, but she avoids talking to him. Asking questions about Perry gets Scott in trouble with the captain and he narrowly escapes being murdered. When Marian disembarks, sailors then try to keep Scott from following her. Scott jumps overboard, and follows Marian to a hotel, but is prevented from getting near her by the staff. After outwitting them, he learns from Marian that she has been summoned by Perry's mother, who lives on a plantation in the interior. Marian refuses to believe Scott's suggestion that Perry is alive and using her. Having been called by the hotel manager, Superintendent Roberts arrives and takes Scott to the police station, where Scott relates his suspicions about the strange things that have happened to him. Although Roberts is skeptical, he knows that Mrs. Henderson lives in town and takes Scott to meet the dotty, tipsy woman. When Roberts and Scott return to question Marian further, she has already left for the jungle with a native group led by Vincent, a guide in the Henderson employ. After hiring his own guide, Scott pursues her. When they catch up to Marian's group, Scott pays off his guide, and intrudes on the camp, claiming he is "John Palmer" and lost from his safari. Although Marian does not refute his story, she insists that he be sent away with a guide the next morning. After Scott tells her about Mrs. Henderson, Marian changes her mind and allows Scott to accompany them, but the kindly Vincent privately warns Scott to leave. The group continues through the jungle, later taking canoes through dangerous waters until they finally arrive at their destination, a village in which the living Perry rules. Although Perry is suspicious of Scott, Vincent and Marian vouch for him, so Perry changes into a condescendingly gracious host. Perry admits that he jumped off the ship and reveals that after he collects the insurance money, Arthur will come to the jungle and switch identities with him. Marian secretly fears that, having confided in Scott, Perry will not allow him to leave alive. Later, Perry takes Scott on a lion hunt and sabotages his gun, but Vincent saves him. When they return, one of Perry's henchmen brings him the news that Scott is an insurance investigator. Instead of killing him outright, Perry allows Scott to leave alone, knowing that he cannot survive poisonous plants, cannibals, quicksand and rapids. Marian urges Scott to go, but first Scott saves Vincent, who is being tortured by Perry for bringing Scott to him. Scott, Vincent and Marian escape together in canoes, but Vincent is shot by Perry and his henchmen, who are closing in from behind. When their canoe nears a huge waterfall, Scott and his companions go to shore. Thinking they have been drowned, Perry turns back until he hears Scott shoot a large, attacking snake, and then resumes the chase. Meanwhile, in Johannesburg, Roberts has had another visit with Mrs. Henderson and sensing trouble, instigates a search for Scott and Marian. He and his men arrive in time to save Scott from Perry, and Vincent and Marian from a lion. Perry escapes in a canoe into the rapids, but his canoe overturns near a crocodile. After saving Perry, Scott hands the villain over to Roberts, and then accepts Marian's affections.
