On the Mariposa, the schooner that is bringing Tex and Kit back to the United States after the adventure in Panama, a group of lawless Englishmen led by Steven Brooke forces Captain Ortega to head for Providencia, a Mexican island. Not even the storm that throws the ship against the rocks discourages Brooke: the hijacking of the Mariposa, with its entire load of arms, is the first act of his mad revolutionary dream: to conquer the island and set up a small independent state there. But a strange scientist is also traveling on the sailing-ship... and another monstrous passenger reawakens with the shipwreck!