William Deep is the youngest, a fearless conqueror of the deep sea - this is how young Billy imagines himself while relaxing with his sister Sheena on the schooner Cassandra, which belongs to his scientist uncle. But this year, reality will surpass any imagination - because the coral reef, near which “Cassandra” dropped anchor, is fraught with many mysteries. There are even rumors of a mermaid living in these waters, and Billy becomes eager to catch her. But an amazing meeting with a guest from the depths will not bring him happiness...