No pun intended.
Alexander is a kid from the wrong side of the tracks -- or in this case, the canal, in the undersea city of Thessalonika. His father is a drug addict and there's never any money for things like food or school supplies. Alexander is smart and could make something of himself, if the stratified society of his world would just give him a chance.
His best friend in school is Jade, heroine of one of Catherine's full-length books. Jade is upper class and her father is an important man. The social unrest that divides the undersea world directly impacts Jade and Alexander, their friendship, and his plans to make a better life for himself. He has to make some painful choices. Will they work out?
That's all I'm gonna tell you. Get the story, read it, and give in to the temptation to buy one of the full-length books and return under the sea. This ain't your granny's Little Mermaid by a long shot (well, at least not the Disney version... Thank goodness!)