In the 21st century, the Earth was captured by aliens whose habitat is water. Earthly civilization survived, but two hundred years later earthlings are forced to sail the seas on sailing ships. Any sea voyage is fraught with a meeting with pirates or crazy sectarians who believe in the coming of amphibians. Roman Naydenov, together with the crew of the Medusa, is engaged in the illegal collection of alien organisms in the Black Sea. He also works part-time as a diver-navigator - he takes those who like to profit from forbidden fruits to places known only to him. During one of these dives, clients began to behave very strangely...Each hero will get a portion of rum and adventure.