THE STORY In response to a letter from Jane, who is in England nursing British troops, Tarzan and Boy trek across the desert looking for jungle plants to be used in creating a malaria serum. On the way they save a Jaynar, a wild horse, from a German, arrive at an Arab city and .rescue a stranded American magician, Connie Bryce (Nancy Kelly), When they reach Bir Herari, they meet Hendrix, who is really the Nazi, Heinrich, whom Connie is on a mission to expose. Heinrich arrests Tarzan as a horse thief. Heinrich and his cronie, Strader, follow Connie who is delivering a message to the sheik's palace. The Germans kill the sheik's son, Prince Selim (Robert Lowery) and frame it on Connie who is sentenced to hang. Tarzan escapes and summons the stallion Jaynar and during the resulting stampede, Tarzan is able to rescue Connie and take her to a nearby jungle where Tarzan fights off prehistoric monsters to obtain the fever medicine. He battles a maneating plant and throws the Nazis to be killed by a giant spider. 

The film went through a number of re-writes and re-shootings which resulted in the finished project being substantially different from what the director  originally started with. The pressbook synopsis gives an idea of what changes were made in the process of shooting:  Tarzan, Boy and Cheta the Chimpanzee come out of their jungle to the fringes of the Sahara Desert seeking a curative herb for Mrs. Tarzan. In the desert they acquire a new companion, a beautiful stallion named Jaynar. They also learn that Nazi secret agents are striving to instigate trouble in the territories of two important sheiks, one of whom sends a beautiful American chorus girl, Connie Bryce, to warn his neighbor against the head agent, Hendrix. The sheik’s son, Prince Selim, also tries to warn his father, but is not believed. Meanwhile, Connie clashes with Hendrix and his henchmen. The clash brings Tarzan and the girl together, and they join forces in a common cause. Before she can deliver her message, however, Prince Selim is killed by the conspirators, who succeed in getting Connie tried for the crime, and condemned to die. Tarzan is also thrown into prison. Boy and Cheta aid Tarzan to escape. On Jaynar the stallion, Tarzan rides through crowds gathered to watch Connie’s execution, and rescues her. They hide in the desert, but the girl is captured by Hendrix, while Boy goes on to seek aid from the neighboring sheik. Tarzan sees this sheik, obtaining from him proof of Connie’s identity and the enemy plot. On his return, he finds Connie again about to face the gallows. He dashes in, confronting the enemy agents with the damning evidence of their guilt in the presence of the sheik. This forces Hendrix’s hand. He gives word for the uprising to start. Boy, meanwhile, is guiding a column of the neighbor sheik’s men to the rescue. Tarzan’s audacity and quick action temporarily saves the situation at the sheik’s palace. He also prevents the insurrectionists from ambushing the column led by Boy. A pitched battle between the combined forces of the sheiks, aided by Tarzan, and the enemy forces, ensues. The latter are routed. Connie bids a fond farewell to her new desert friends and departs for London, bearing with her the medicinal herbs for Mrs. Tarzan.