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.
A scientist and his nephew are hosts of Lana, queen of the Amazons. Meanwhile, other white men penetrate the Brezilean jungle, wishing to find and take hold of the legendary Amazons' treasure.