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 on a trip to St. Louis, fourteen-year-old Trixie Belden discovers some mysterious papers in her hotel room and soon realizes that she and her friends are being followed as they cruise in a towboat down the Mississippi River.