Based on the adventure fiction novel "The Amphibian Man" by Alexander Belyaev
A small seacoast town is full of rumors about a 'sea devil.' A wealthy merchant, Zurita, who trades in pearls, does not believe in it until he runs into the 'devil' under dramatic circumstances. After that he is eager to get this 'devil' and use him for his own purposes. The 'devil' is an ordinary, or, to be precise, extraordinary young man. When he was a small child a doctor saved his life by transplanting shark's gills into the boy's body. As a result, the boy, Ichtiander, can live both on land and in water. Ichtiander falls in love with Guttiere, the girl Zurita is going to marry, and becomes Zurita's sworn enemy and prisoner…