Plot summary

Their SOS ignored by a strange yacht in a storm, the Hardy Boys find a wallet, apparently picked up by a helicopter, containing two thousand dollars alongside their speedboat and are launched into a mystery involving diverse clues. This includes finding out who kidnapped Jack Wayne and took his plane, and where the wailing siren is coming from.

Original edition (1951)

• Ghostwriter: Andrew E. Svenson, rewritten by Harriet S. Adams
• Summary: The boys discover that Central American revolutionaries are hiding stolen equipment and munitions in the North Woods of Bayport.

Revised edition (1968)

• Revision type: Minor revisions.
• Ghostwriter: Priscilla Baker-Carr