"The Princess in Black and her friends are enjoying a day of sun and sea on Princess Sneezewort?s royal boat when a real, live mermaid princess emerges from the waves! Eeeeeee! Princess Posy needs their help protecting her very cute sea goats from being eaten by a very greedy kraken. But the princesses and the Goat Avenger quickly realize that fighting underwater can be tough for land dwellers, and only the mermaid Princess Posy can save the day....

Eleven-year-old Princess Celie lives with her parents, the king and queen, and her brothers and sister at Castle Glower, which adds rooms or stairways or secret passageways most every Tuesday, and when the king and queen are ambushed while travelling, it is up to Celie--the castle's favorite--with her secret knowledge of its never-ending twists and turns, to protect their home and save their kingdom.