Spirulina's father, Neptune, has lost a competition to Ariel the air sprite and must hand over all his treasure. But there's worse to come. The treasure has mysteriously disappeared, and if it can't be found, Neptune will be forced to give up his home to Ariel. Coralie and Shelle can only wail and cry at the thought of their father losing the beautiful Floating Palace, but Spirulina is determined to do something about it.