The melted coins.

Sinopsis(en Inglés)

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Frank and Joe Hardy suspect that their best friend Chet Morton is the victim of a summer school swindle and offer to help get his money back. While probing a baffling burglary at the Seneca Indian Reservation in New York State they investigate Zoar College located nearby. Clues that Frank and Joe uncover indicate that there is a connection between the Zoar College swindle and the theft of the Senecas' gold tribal relic Spoon Mouth. This startling discovery propels the teen-age sleuths into a series of perplexing and dangerous situations. Two strange-acting college professors, a valuable coin collection, and a taciturn Indian who refuses to discuss the mysteries surrounding Spoon Mouth -- all blend into a fast-paced story with several surprise twists that will thrill the vast legion of Hardy boys fans. - Flyleaf.