Danny Quinn lives with one great dream....he wants to prove his late father was right about a pirate treasure in the depths of Lake Michigan.
Everyone opposes Danny's dream, especially his grandfather, Ben, a practical old salvage diver who doesn't believe in such nonsense. Suddenly everything changes though when Ben has a tragic accident during a dive on a local wreck. The bank forecloses on Ben's property, and Danny, faced with living in a foster home, runs. That night he meets the ghost of Picaroon, who has come to warn Danny about a mysterious foe and to help him find the gold.
Danny sets off on his great quest, taking a ragtag crew which includes Toby Weems, a windy old town character, Rufe, Danny's oddball buddy, and Smokey Jeanne, a tough Chicago girl. They sail off pursued by treasure hunter, Doc Biehler, and his weird wife Mother. After a knock-down battle with Doc, who makes off with the treasure map, it's Smokey's toughness and support that wins Danny over. Eventually, Danny must battle Doc for possession of the map. The final twist is learning that Doc's wife, Mother, is the real brains behind Doc's group and a master swordfighter. But Picaroon's training pays off and Danny defeats Mother to win the treasure.
At the end in an emotional farewell, Picaroon returns for one last visit. Then, after seeing the treasure, sails off, leaving Danny and his friends jubilant that the Quinn family name will be restored and that all debts will be settled.