The ownership of the island was granted to an American soldier of fortune named Dirkson after the First World War by the Royal house of Sulu.

The ownership of the diversified plantation island would be perpetually owned and ruled by the Dirkson family in a manner similar to the British Brooke family, who founded and ruled the Raj of Sarawak.

An international financier named Bellflower (Torin Thatcher) and an Indonesian Prince Ali Akeem (Vic Diaz) seek El Dorado for themselves, with the former offering large amounts of money to the current owner James Dirkson (George Montgomery's real life brother Jim Montgomery), whilst the latter wishes to seize the island using an army of guerillas and pirates.

When the insurgents murder James Dirkson, his brother John, a tough disreputable soldier of fortune (George Montgomery) avenges the murder of his brother and defends his own private island.

He also is pursued romantically by Bellflower's daughter Marjorie (Julie Gregg) and Maria Vargas, the nurse running El Dorado's infirmary.