When the Royal Secretary of the Island of Kinkou arrives to inform fraternal twins Brady and Boomer of their royal lineage, their lives change drastically. Now, the boys must relocate and claim the throne as joint Kings of the island.

The island's volcano has become active and the natives panic, prepared to abandon Kinkow forever. Attempting to send them to their deaths, Lanny tells the twins of another sacred ruby on the dangerous side of the island. For the good of their people, Brady and Boomer face their fears to obtain the second ruby and save the island from destruction.

Hibachi, the best (and shortest) surfer on Shredder Beach bans Boomer and Brady from the beach after they tick him off. When efforts at befriending Hibachi fail, the boys end up challenging him to a Surf-Off, forgetting that they don't know how to surf.

When Brady and Boomer meet some mermaids on the beach, Mikayla warns the guys that mermaids are bad news. But they don't listen and end up getting locked out of their own castle when they invite the mermaids over.

The twins discover a cute little animal and decide to make him their pet. Unbeknownst to the boys, their pet, Pupaley, grows into a towering behemoth when angered. When the boys try to return him to the wild, he grows and makes they boys HIS pets.

When Boomer and Brady realize that their royal privilege doesn't excuse them from having to go to school, they find themselves back in the classroom, where Brady discovers major competition for Mikayla's affection.