The giant submarine Bluedeep has been plowing the waters of the dangerous ocean of the planet Almoa for ten years. The submarine's sailors include not only adults—teenagers also serve here. Once upon a time they were taken from the shelters of the Earth and sent on a distant expedition, so that years later they would become the new crew of the submarine. But for many years none of them has grown up, not even for a single day... Sigrid is an ordinary third-rank watchwoman, she is only twelve. True, she has been twelve for a very long time. And all this time, her main duty is to ensure that alien sea creatures do not damage the ship’s hull. You especially need to be wary of huge octopuses that can flatten it into a flat cake. But one day, in the closed area of the submarine, the girl found a mysterious cabin and from that moment the secrets of the “Bludip” began to reveal themselves to her one after another...