Quick recap
Billy and his sister Sheena spend the summer on the Caribbean island of Ilandra, aboard their uncle Dr. D's boat. Billy hears a zoo is offering a million dollars for a mermaid someone spotted nearby and sneaks out before dawn to find her. A hammerhead nearly kills him, and the mermaid saves him. His uncle locks her in a glass tank, but assistant Alexander joins thugs who want her for themselves. They toss the family into the tank and sink it. Other mermaids lift it back up, the family breaks free, and the mermaids rock the thugs' boat. Dr. D lies to the zoo. Billy goes back to the reef to say goodbye and gets attacked by a sea monster.
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