On a hot night, the schooner Medusa was anchored off the coast of Argentina. Pearl catchers rested on her deck. Balthazar, an Araucanan Indian, first assistant to the captain and owner of the schooner Pedro Zurita, carried the night watch. In his youth, Balthazar was a famous pearl catcher. Aged, he opened a shop of marine rarities and began working for Zurita.

Balthazar had already begun to doze when he heard the musical sound of the trumpet, accompanied by a cheerful and young voice. Fishermen and pearl catchers were alarmed - it was a sea devil. This unknown creature has long terrorized the coast, helping one and harming others. He cut the nets, threw the fish into the boats of the poor and had fun, poking fun at the fishermen. Scientists could not classify this creature, since no one saw him. Zurita still still did not believe in a sea devil.

In the morning it was discovered that the ropes of the boats tied to the schooner were cut with a sharp knife. A little later, one of the divers saw the devil himself - a creature with scaly skin, huge eyes and frog legs. The devil saved the diver from the shark. Zurita again did not believe it, but soon he himself saw a strange creature sitting astride a dolphin and blowing into a large shell.

Having convinced himself of the existence of a sea devil, Zurita decided to catch him and make him work for himself. Balthazar undertook to help him. The devil appeared again only after three weeks. Following him, Balthazar discovered an underwater cave in which the creature was hiding. Strong nets were placed around the cave, but the devil caught in them managed to cut the ropes.

Zurita did not give up. He filled the bay with traps and nets, but the devil no longer appeared. Finally, Zurita bought two diving suits and he and Balthazar went down to the cave of a sea devil. The cave turned out to be half filled with air, and in its depth a strong lattice with a tricky castle was discovered. After scouring the neighborhood, Zurita stumbled upon a stone wall, behind which was the house of Dr. Salvator.

In Buenos Aires, Zurita found out that the doctor is famous for his bold operations. "During the imperialist war, he was on the French front, where he was engaged almost exclusively in skull operations." After the war, Salvator returned to Argentina and took up science. He treated only the Indians who considered the doctor a god. Zurita realized that Salvator was somehow connected with the sea devil.

Once, an old Indian Cristo (Christopher) came to Salvator with a sick granddaughter. The doctor cured the girl. Out of gratitude, Cristo wanted to devote the rest of his life to the doctor. Salvator “reluctantly and carefully took new servants”, but there was a lot of work, and Cristo got into the doctor’s house. At first, the Indian worked in an external garden enclosed by two high walls. Many strange animals lived there: two-headed snakes and rats, sparrows with the head of a parrot, llamas with horse tails and talking monkeys. This garden was served by very silent blacks.

Soon, Salvator gathered in the Andes for new animals for experiments, and was about to take Cristo with him. He asked to see his family, but actually went to his brother Balthazar. Zurita's assistant sent his brother to the doctor's house to find out about the sea devil. Upon learning of the expedition to the Andes, the brothers made a plan: Salvator will be captured by “bandits”, and Cristo will save him, after which he will become a doctor's confidant.

The plan was a success. Returning home, Salvator brought Cristo to the inner garden. The doctor drained the pool, dug in the middle of a small garden, and went down into the hatch. A long passage led them into the hall with a huge aquarium that went directly to the seabed. A humanoid creature with large bulging eyes and frog legs emerged from the aquarium through a special chamber. The body of the unknown sparkled with a bluish-silver scale. ” Eyes turned out to be glasses, paws - gloves, and scales - a special heavy-duty suit. Underneath all this was a handsome young man named Ichthyander.

The young man called the doctor his father, but did not look like a white man. With regular facial features and dark skin, he resembled an Araucan tribe. Ichthyander could live under water - Salvator implanted in his body the gills of a young shark. It was his fishermen who were considered a sea devil.

Ichthyander spent most of his time in the ocean with his friend Dolphin Leading. Once he saved a beautiful girl. She was unconscious and drifted in the ocean, tied to a board. Ichthyander carried the girl ashore. Seeing that she was recovering, the amphibian man hid - he did not want to scare her. Soon next to the girl appeared "dark-skinned man with a mustache and goatee, with a wide-brimmed hat on his head" and posing as his savior. Ichthyander was amazed and indignant at such a blatant lie.

Cristo became a servant of Ichthyander. In the air, the young man could spend a little time: when the gills dried up, the young man began to choke. It was Cristo's responsibility to ensure that Ichthyander slept not a few nights a week in water, but in a regular bed. Ichthyander received a good, but too one-sided education. He was well acquainted with the natural sciences, but knew practically nothing about life on land. The young man understood matters of everyday life worse than a five-year-old child.

Meanwhile, Salvator again went to the mountains. Ichthyander could not forget the girl he saved, and Cristo managed to lure him into the city, promising to look for a beautiful stranger. Ihtiandra did not like the hot and dusty city. Cristo brought him to Balthazar's shop. While the brothers were talking, Balthazar's adopted daughter, Guttiere, entered the room. The girl was famous for its beauty and inaccessibility. Seeing her, the young man jumped up and ran away - he recognized her.

Some time later, Ichthyander himself came to the Balthazar shop. On the shore, he saw Guttiere, who passed a pearl necklace to a tall, broad-shouldered man named Olsen. Suddenly, the necklace slipped out of the girl's fingers and fell into the ocean. The place there was deep, and the necklace would have been lost, but Ichthyander pulled it out. So the young man met with Guttiere. Now they met almost every evening. Balthazar did not suspect that the new gentleman of his daughter was the sea devil.

Once Ichthyander returned home wounded - he saved Leading from the hunters. Bandaging the wound, Cristo saw on the shoulder of a young man a large dark mole of an unusual shape. Despite the wound, Ichthyander went on a date with Guttiere. Suddenly a horseman drove up to them, which the young man recognized: it was a man who had impersonated the girl’s savior. The rider, who turned out to be Zurita, said that the bride should not walk around with others on the eve of the wedding. Learning that Guttiere was someone's bride, Ichthianndr began to choke, and rushed off the cliff into the ocean. Guttiere decided that the young man she liked so much drowned. Balthazar again tried to persuade his daughter to marry Zurita, but in response he again heard "no."

After spending several days at sea, Ichthyander yearned. He found Olsen's boat in the sea. He told the young man that he was not the bridegroom of Guttiere at all, and that most recently the girl had become Zurita's wife - he had taken her by force. Guttiere and Olson were about to escape to North America, but did not have time. From Olsen, Ichthyander learned that Zurita had taken the girl to his Dolores hacienda, and decided to go after her.

Ihthyander did half the journey to the hacienda along the river. Further it was necessary to go on foot. The young man was not lucky: on the way he met a policeman, whom the crumpled suit of Ichthyander seemed suspicious. A murder occurred on a nearby farm, and the policeman decided to accuse a suspicious young man of it. He handcuffed Ichthyander and led him to the nearest village. Once on the bridge over the pond, Ichthyander jumped into the water and pretended to be drowned. While the police were looking for a “drowned man,” Ichthyander reached the hacienda.

Once in place, the young man tried to find Guttiere, but came across Zurita. He hit Ichthyander on the head and threw it into the pond. Guttiere heard fuss in the garden, went out to the pond and saw a man whom she considered dead coming out of the water. Ichthyander admitted that it was he who was considered a sea devil. Zurita watched the young wife vigilantly and managed to eavesdrop on this conversation. He realized that the sea devil was finally in his hands. He saw that Ichthyander was handcuffed, and threatened to hand him over to the police. Guttiere began to beg her husband to spare Ichthyander, and he pretended that he could not resist the pleas of his wife. He promised to transfer Ichthyander to his schooner and release it in the open ocean. However, once on the schooner, Zurita locked Ihthiander in the hold, and Guttiere in the cabin.

Meanwhile, an important conversation took place between the brothers Cristo and Balthazar. Balthazar's wife died during childbirth when Cristo ferried her across the mountains. Then he told his brother that the child also died. In fact, Cristo carried the boy to Dr. Salvator, who said that he could not save the child. By birthmark, Cristo recognized his nephew in Ichthyander. The news that his son was alive and became a sea devil struck Balthazar.

The next day, Salvator returned from the expedition. Cristo informed him that Ichthyander was kidnapped. The doctor rushed to save the young man in a submarine hidden in a cave under the house.

In the morning, Zurita ordered Ichthyander to be put on deck. The young man was not feeling well. He could not live without clean water, but he had to plunge into a barrel from corned beef. Stunning the "sea devil", Zurita shackled him in a metal belt on a long chain and sent him to look for pearls, promising to let him go after that. The catch of Ichthyander struck Zurita. He wanted more, but was afraid to release an amphibian man into the ocean without a chain. Zurita decided that he could keep the young man with the help of Guttiere, but she refused to help him.

Meanwhile, the schooner team found out that there was a sea devil on board, and rebelled. The sailors decided to kill Zurita. Escaping, he climbed onto the mast and saw a doctor’s submarine approaching the ship. Frightened sailors jumped into the water. Zurita also left the ship, capturing the stubborn Guttiere. Ichthyander was not on the abandoned schooner. Salvator did not know that the young man was under water. Not far from this place a rich passenger ship sank. Zurita forced Ichthyander to go down there and collect all the jewelry, showing him a fake note from Guttiere. The naive young man complied with the villain’s order and was already sailing to the shore when the girl managed to shout so that he could escape. Ichthyander decided to sail away from people.

Balthazar, meanwhile, did not find peace. He wanted to tear his son out of the clutches of Dr. Salvator, who seemed to him a monster. He found a crook-solicitor who, on behalf of Balthazar, sued the doctor. A lawsuit was instituted against Salvator and Zurit - he wanted to take possession of Ichthyander forever, becoming his guardian. The trial turned out loud, because the prosecutor and the bishop were against the doctor. Examining the house of Salvator and seeing the results of his experiments, scientists considered the doctor a brilliant madman. For himself, the doctor was calm. He was worried only for Ichthyander, who was also kept in prison. At the trial, Salvatore said that he wanted to create people of the future who could live in the ocean and use its inexhaustible resources.

In prison, Ichthyander had to live in a smelly iron barrel and eat raw fish. Such a life led to the fact that the young man was almost unable to breathe air. "The amphibian man turned into a human fish." Fortunately, the prison governor owed a lot to Salvator - the doctor saved his wife and child. He found out that Ichthyander was wanted to be killed as a “godly creature” and decided to save him. Olsen took out an amphibian man from prison. Once in the ocean, the young man sailed to the Tuamotu Islands, where Dr. Salvator's friend lived. The doctor himself hoped to get out of prison in a few years and reunite with his adopted son.

Guttiere saw Ichthyander sailing into the ocean. She did not dare to appear in his eyes, afraid that the young man would refuse to swim away. The girl ran away from her cruel husband, moved to New York and married Olsen. Cristo stayed to serve with Salvator, who was released from prison and was preparing for a long journey. Only Balthazar, who was considered crazy in the city, remembered the sea devil.