Sea-devil has appeared in the Rio de la Plata. Weird cries out at sea, slashed fishermen's nets, glimpses of a most queer creature astride a dolphin leave no room for doubt. The Spaniard Zurita, greed overcoming his superstition, tries to catch Sea-devil and force it to pearl-dive for him but fails.
On a lonely stretch of shore, not far from Buenos Aires, Dr. Salvator lives in seclusion behind a high wall, whose steel-plated gates only open to let in his Indian patients. The Indians revere him as a God but Zurita has a hunch that the God on land and the devil in the sea have something in common. Enlisting the help of two wily Araucanian brothers he sets out to probe the mystery.
As action shifts from the bottom of the sea to the Spaniard's schooner The Jellyfish and back again, with interludes in sun-drenched Buenos Aires and countryside, the mystery of Ichthyander the sea-devil is unfolded before the reader in a narrative as gripping as it informative.