The Princess in Black and her friends the Princess in Blankets and the Goat Avenger are enjoying a day of sun and sea when something springs from the waves. It's a mermaid princess! The Princess in Black has always longed to meet a mermaid. Princess Posy needs the masked friends' help protecting her sea goats from being eaten by a greedy kraken. But the heroes quickly realize that fighting underwater can be tough and only the mermaid princess herself can sve the day. Can they help their new friend learn that there is more to being a princess than being nice?