Dan Thunden was the only survivor of a shipwreck that took place in 1843, somewhere in the Caribbean. He managed to survive for nearly a century on the uninhabited island by drinking a tea made from a rare herb. Thunden was eventually rescued by Santini who was fleeing from South America to the United States in an airplane.
Apparently, Santini landed at the island to make repairs to his airplane and found Dan Thunden, who told Santini his story. A partnership emerged with plans to market the miracle herb to wealthy men in the belief that they would achieve immortality.
Santini double-crosses Thunden, which results in Doc Savage eventually being dragged into the sequence of events, when Santini orders Doc kidnapped so as to keep him out of the way for a few weeks.
Fear Cay also features the second appearance of Doc's cousin, Patricia Savage, who shows up looking for adventure and gets more than she bargained for as she is mistakenly kidnapped by Santini and his gang and swept away to the island, Fear Cay.