In 1946 a young German widow and her two sons arrive in Venezuela to begin a new life. Helga will marry her brother-in-law, Klaus, a doctor, and the boys, Erich and Zeppi, come to accept him as their stepfather. Together the family fly out to a distant jungle outpost where Klaus is to be the new medical officer, but en route their plane flies into a fierce tropical storm and crashes into a river in a remote region of Amazonas.
They are rescued by the Yayomi, a tribe of stone-age Indians, who assume they are terrestrial incarnations of magical dolphins and accept the family into their tribe. They are not alone among the natives: Gerhard Wentzler is an anthropologist who has been studying the Yayomi for years, too long to be able to leave.
At first all goes well, the boys in particular adapting readily to Yayomi ways, but slowly and painfully Erich begins to realize that Klaus may not be the noble figure he thought he was.