Just outside Mogamba, native expert and guide Jungle Jim and several soldiers raid a camp of spies at the behest of Major Walsh. Upon reporting to Walsh in Dangor, Jim is introduced to Dr. Parker, a radiological expert, who explains that the Anglo-American governments wish to conduct the first African atom bomb test, known as Jungle Project X, and have been waiting for the elimination of all foreign spies. Now cleared to proceed, Parker selects the island of Tulonga as the ideal site for the test. Jim's long association with the highly superstitious Tulongans makes him the best choice to oversee their removal from the island to the mainland. When orders are given for all traffic to cease between the mainland and Tulonga, traders Carl Kroman and Emil Bruno protest to Walsh. Unknown to Walsh and Jim, Kroman and Bruno are foreign spies who tipped Jim off to the other agents to deflect attention from themselves. Kroman is blackmailing Parker's assistant, Paul Benek, into providing them with decoded Project X messages from the government. Kroman intends to force the Tulongans to remain on the island to cancel the test. Walsh assigns Joan Harris, a representative from the World Health Organization, to travel to Tulonga with Jim to vaccinate the natives and assist in their transfer. After a harrowing journey to Tulonga, Jim and Joan discover Kroman and Bruno on the island, trading with Chief Wamai. Kroman protests that they are simply making a farewell trade, but secretly provokes native Lutembi by insinuating that their trade will not go through should they leave the island. Lutembi argues with Wamai and instigates a mutiny among the natives, and challenges Jim to fight. When Jim successfully knocks Lutembi out, the tribe agrees to the transfer after receiving their inoculations from Joan. Along the path to the river, Kroman and Bruno plant several masks and animal carcasses, knowing the Tulongans will view them as evil omens. After the natives come upon the items, Lutembi refuses to allow the tribe to continue until Jim proves the path is safe. When Jim investigates ahead, Kroman releases a panther, which attacks Wamai, who is saved by Jim's hasty return. Lutembi insists that the attack is a sign from evil spirits, but Jim demands they wait and see if Wamai recovers. He and Joan work on the chief through the night and by morning Wamai is recovered enough to travel. Unable to halt the native evacuation, Kroman and Bruno return to the mainland to make additional plans. As the natives settle into their new camp near Dangor, Benek brings Kroman another decoded message informing Walsh and Parker that Project X is proceeding as planned. Kroman decides to use a new invention that stores uranium by-product in a dust form, which he will spray over the native camp, causing illness among them and provoking their return to Tulonga just before the bombing. With photographic evidence of the destruction of the Tulongans, Kroman intends to sabotage the Anglo-American bomb tests around the world. Early the next morning, a crop duster douses the native camp with the uranium dust, and soon Joan receives word of illness spreading among them. Lutembi immediately insists the tribe return to Tulonga, but Wamai agrees with Jim to try Joan's medicine. Stumped as to the cause of the natives' illness, Joan and Jim report to Parker, whose geiger counter begins clicking near them. Parker speculates that somehow the natives have been contaminated. As Parker and Joan head off to the native camp to investigate further, Jim goes to Benek's hut to retrieve new messages, but learns that Bruno has just taken Benek to Kroman. Jim follows and after overhearing Benek protesting about the planned murder of the Tulongans, breaks in. Before Benek can confess the truth, Bruno knifes him and attacks Jim. Kroman sets the hut on fire, and he and Bruno escape as Jim helps the wounded Benek. Kroman and Bruno hurry to the native camp, where they try to persuade Wamai and Lutembi that Jim has lied to them and urge them to return to their island. When Parker and Joan protest, they are tied up and the tribe makes plans to depart. Jim takes Benek to Walsh, and Benek confesses everything. Walsh intends to cancel the test but discovers that Kroman has destroyed the only radio in camp. Jim assures the major he will stop the Tulongans from leaving the mainland and races down the river in a small canoe. He is spotted by Kroman and Bruno, who try to stop him, but Jim kills them both before arriving at the river mouth ahead of the Tulongans. There Jim releases the natives' canoes. After a furious Wamai and Lutembi arrive, he promises them that if the giant fire god he told them about does not consume their island that morning at sunrise, they can kill him. Joan and Parker leave, and soon after sunrise, the natives witness the devastating power of the atomic bomb explosion while Parker, Jim and Joan watch with relief.