Jake Long is disappointed he has to go to Aunt Cathy's private island in the Florida Keys for a family reunion. Mainly because he always ends up at the kids table with his other younger cousins. But this year, he has some hope since his Uncle Lee had his appendix removed and cannot make it this year.
While Susan, Haley, and Jake fly to Florida, Gramps and Jonathan drive instead. The point is to keep Jonathan from going to the reunion, since Susan's side of the family is so sloppy at keeping their magical secret a secret.
Once at Aunt Cathy's, Jake is once again disappointed he is stuck at the kiddy table. But even more angry because the available seat was given to his one year younger than him cousin Gregory, for just now getting his dragon powers. Jake tries to confront him, but they agree on a one-on-one battle against each other; winner gets the spot at the adult table.
The next morning, Jake and Greggy do a best 2 out of 3 dual. The first round Jake won, and by technicalities, Gregory won the second one. However, the last competition chosen by Gregory was to take down a helpless shark lady that needed help, but when Jake refused to fight her, Gregory ended up winning.
Later on, Jake goes out to sea to find that distressed shark lady, and to get away from his family. Out in the ocean, they get confronted by other shark people, looking for something. But when Jake escaped, that is when he finally found the shark woman.
She went on and told them about Neptune's Trident, the object Tiberon and his shark gang was after. With the powerful trident, they could control the seas. Luckily, the shark lady new where it was at, so Jake and Fu agreed to help her find a place to hide to make sure it was safe.
The three head back on the island, so Jake could tell his mom what he was doing. But unfortunately, the shark lady made a mess eating the family's dinner, and also scarred Greggy. With Jake now happy, they were headed off the island. But, it wasn't long before they were all captured by Tiberon. Tiberon then grabbed the shark lady, and forced her to gag up the trident. He then escaped out to sea.
Jake and Greggy go to follow them, and together (mainly just Jake), they defeat him and get a hold of the trident.
Back on the island, Gramps and Jonathan finally make it to the reunion when a fight against the family breaks out. Jonathan intervenes, saying this isn't how family is should act. But they all find a way to set apart their differences, enjoy somewhat of what's left of their reunion.