Oona is a mermaid who loves to explore and seek treasure.  There is a not quite buried treasure that she has wanted for some time, but the obstacles to achieving this goal are many.  However, Oona has the help of her friend Otto the otter.  There are parts of this book that I see holes in as an adult reader.  Why the need for the special glasses, and who made the bridge for what reason.  But most of that will fly by the children.  What they will see is a beautiful black mermaid and her fuzzy soft friend having adventures and living a glorious life.