When Rocky finds a seaweed scroll that describes an abandoned pirate ship bursting with jewels, the students are tripping over their tails with excitement. There's only one problem: the treasure is supposedly guarded by pirate ghosts! But a few "silly ghosts" aren't going to keep prima donna Pearl away from fame, fortune, and "diamonds as big as a merman's fist."

The hunt is on! Pearl recruits fellow merkids Rocky and Wanda to help her track down the treasure by swimming all the way past Whale Mountain. But when they discover the pirate ship, they hear eerie moans and groans coming from within. What's making those sounds? Could the pirate ghosts be real? And can the merkids get the jewels before becoming someone's--or something's--lunch?