Set in the Soviet scientific community, the story follows two dedicated physicists whose close friendship unfolds amid dangerous nuclear research, shaped by their shared affection for a strong-willed woman. Radiation exposure, scientific ambition, and romantic rivalries intersect as the characters confront the physical risks of their work and the personal consequences of their choices.

People living at a seashore town are frightened by reports of an unknown creature called "the sea devil". Nobody knows what it is, but it's really the son of doctor Salvator. The doctor performed surgery on his son and now young Ichtiander can live under water. This gives him certain advantages, but also creates a lot of problems.