American engineer Robert Warren enters Huerta's International Café, a bar along the Amazon River, and demands the money owed him for building the air-conditioning system. Huerta, the owner, goes into his office and is attacked by an unseen assailant, while another man, Victor Sloma, searches through his files. Impatient for his money, Robert comes into the office and finds Huerta dead. One of the bar girls then enters and accuses Robert of murder, and the police pursue Robert to the docks and open fire. Robert is shot in the shoulder, but he escapes the police by diving into the water and climbing aboard a steamship, where he hides in a cabin occupied by Victor and Maria Marek. After Victor leaves the cabin, Robert asks Maria for help, and as she bandages his wound, he tells her about the murder. Maria tells him that the boat is going to her husband Gregory's plantation in the jungle, and that Victor is Gregory's partner. The next day, Robert tries to speak to Maria of his feelings for her, claiming that they were fated to be together, but she refuses to listen. The boat docks, and while Maria and Victor greet Gregory and Damiao, his mute servant boy, the sailors find Robert and throw him overboard. Later, at Gregory and Maria's hacienda, Victor assures Gregory that Huerta will no longer blackmail them, adding that an American working for Huerta was accused of the murder and shot by the police. Meanwhile, Robert makes his way to a jungle clearing and finds work with the plantation's crew. During dinner, Robert learns that Huerta used to be an overseer on the plantation before making his fortune during the war. The natives also mention that some time before, Señor Blanco, who used to be Gregory's boss, fell into the river and was devoured by piranhas. Later that night, one of the natives is bitten by a snake, and despite Robert's efforts to save him, the man dies because they have no serum on hand. Angry at the man's needless death, Robert seizes the overseer's gun and orders him to go back to the island for medical supplies. When Victor hears that the overseer has an American on his crew, he anxiously consults with Gregory, who decides that they will deliver the supplies themselves. After speaking with Robert, Victor proposes getting rid of him immediately, but Gregory decides to take Robert back to the plantation, and at dinner, he tells Maria that he has commissioned Robert to build a hospital in the jungle. After dinner, Robert declares his love for Maria, and later that night, Maria tells her husband, who pretends to be asleep, that she does not love him anymore. The next day, while Robert is working on his blueprints, Damiao shows him some drawings that he secretly made in Robert's notebook. Robert gives him some paper and pencils, and when Victor sees Damiao drawing, he becomes concerned that the boy has found a way to express himself. Gregory tells Victor to take Robert up the river, as he did with Blanco, but as Robert prepares to leave, Damiao becomes agitated and draws a picture of a fish eating a man in the water. When they stop at the wheelhouse, Victor lunges for Robert, but he steps aside and Victor plunges into the piranha-infested river. Meanwhile, at the hacienda, Maria admits to Gregory that she has fallen in love with Robert, but he refuses to let her go. Later, as the badly injured Victor is brought back to the plantation, Robert accuses him of murdering Huerta, and before he dies, Victor confesses to the crime, explaining that Huerta was blackmailing Gregory about Blanco's death. Robert confronts Gregory, and the two men fight, and just as Gregory is about to stab Robert with a pair of scissors, Damiao shoots and kills him. Maria comes out of the hacienda, and she and Robert join hands.
