Plot summary (original edition)

A case involving counterfeit money, buried treasure, and the Curse of the Caribees, Frank and Joe solve the mystery of the Melted Coins and help shut down both a local counterfeiting ring and a much larger operation dealing in stolen gold.

Plot summary (revised edition)

Frank and Joe Hardy become suspicious when their friend Chet Morton enrols in a summer school that sounds too good to be true. While investigating a burglary at the Seneca Indian Reservation in New York State they also investigate Zoar Collage which happens to be nearby. Their sleuthing leads them to believe that there is a connection between the professors of what appears to be a fictional collage and the theft of a tribal mask titled Spoon Mouth. This mask was created when some melted coins happened to look like a sacred Indian image. Now it is missing and the boys' investigation proves to be dangerous however they successfully recover the mask and capture the thieves.

Original edition (1944)

• Ghostwriter: Leslie McFarlane
• Summary: After receiving an amnesiac's warning about the "Curse of the Caribbees," the boys foil a gang that is stealing valuable coins and melting them down for counterfeit.

Revised edition (1970)

• Revision type: Completely rewritten, with no plot elements retained from the original; the new story involves a summer-school scam and the theft of a sacred Seneca mask.
• Ghostwriter: Andrew E. Svenson