Plot summary

Frank, Joe, Chet and Tony travel to Puerto Rico to investigate the mystery behind a coded letter they received from the Hardy father. But with danger following their move and man who has an uncanny resemblance to Joe, the boys are running out of time! Can they save their father before it's too late?

Original edition (1957)

• Ghostwriter: James Duncan Lawrence
• Summary: A cryptic message from their father brings the boys to the Caribbean, where they foil a plot involving a Cuban revolution and an atomic bomb.

Revised edition (1966)

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