The book tells a story of a young man named Ichtiandr (literally "Fish Man" in Greek) who as a child received a life-saving transplant - a set of shark gills. The operation was performed by his father, a scientist and a maverick surgeon, in an attempt to save his son's failing lungs. The experiment was a success but it limited the young man's ability to interact with the world outside his ocean environment. Similar to other Beliaev's works, the book investigates the possibilities of physical survival under extreme conditions, as well as the moral integrity of scientific experiments. It also touches on socialist ideas of improving living conditions for the world's poor.