The Devil Came from Akasava (1971) - Jesus Franco

British scientist Prof. Walter Forrester has found an intensely radioactive mineral in the Akasava jungle of South America that can purportedly transmute elements. His assistant collects a specimen in a cave, but it is stolen and the assistant suffering from a gunshot wound. When Forrester goes for medical help, he vanishes. Given the strategic value of the mineral, Scotland Yard joins the case. Their detective is murdered while searching Forrester's office in London. They dispatch Jane Morgan, an attractive agent, to Akasava to investigate. She meets and eventually joins forces with Rex, the professor's nephew who is also concerned of his fate. Someone with inside knowledge is at work. Suspicions land on Dr. Andrew Thorssen and his flirtatious wife Ingrid. Someone who also in heavily invested in finding Dr. Forrester is his friend Lord Kingsley. Finally, an important mental patient under the close but secret supervision at the asylum of Dr. Henry.