Ever since her father was lost at sea, Sophia has been moping around Aqualand, the marine theme park her dad and uncle created. But Sophia's world is turned upside down when an "aquanaut" breaks into the park's research lab.

To her amazement, Sophia discovers that the aquanaut is not what it seems -- inside lives a band of four goofy sea creatures! And when they all realize that Aqualand has evolved into something much darker than Sophia's dad had envisioned, Sophia is determined to help the aquanaut crew free the park's captive marine life before it's too late.