While preparing for a fund-raiser antique show, Trixie and the other members of the Bob-Whites discover a secret code in the pages of an old magazine in the attic.
When the Bob-Whites--minus Dan--head to Cobbett's Island to spend ten days in a historic home on the beach, Trixie finds an old letter tucked away in a book and sets off on a long-delayed treasure hunt.
The Bob-Whites, plus their friends from Iowa, add detective work to their New York City sightseeing trip after Trixie helps a stranger and receives, as thanks, a purse containing a cryptic prophecy and a promise of great fortune.
Thirteen-year-old Trixie Belden and her friend Honey make several trips to the local marsh in search of herbs and flowers to replace their teacher's plant specimen collection and to look for the pirate treasure that is rumored to be hidden there.
While organizing an ice carnival to raise funds for her pen pal's school library, Trixie investigates a mysterious problem facing Regan and a run of thefts and accidents in the neighborhood.