When Electra hears of hidden treasure in the caves near Mermaid Island she is very excited. She and her mer-friends decide to form a treasure hunters club but their first hunt is abandoned when everyone gets stung by purple jellyfish! On the way home Electra finds a tiny rare dwarf seahorse surrounded by the same jellyfish. She swims in and saves him before he gratefully swims off to his secret seahorse lair.

No one wants to explore very far after being stung and Electra gets very frustrated - in fact she and Sasha have a big fight about it. Sasha is so upset that she swims off - in the wrong direction! Electra swims after her but before they realise it they're both lost. Luckily the tiny seahorse appears again to lead them back to safety and on the way he leads them to a secret cave with a very special surprise!