Footprints Under the Window (Hardy Boys, Book 12)

A plot to steal a top-secret instrument vital to the United States space program poses a challenging case for Frank and Joe Hardy. After rescuing a South American stowaway who mutters a warning about “Footprints “ and then vanishes, Frank and Joe discover that documents belonging to their famous detective father are missing and that footprints linger under a window. The young detective’s search for the diabolical mastermind of the “Footprints” spy ring takes them on a flight to a group of islands off the coast of South America.

• While the Clock Ticked (Hardy Boys, Book 11)
• The Mark on the Door (Hardy Boys #13)
• The Hidden Harbor Mystery (Hardy Boys #14)
• What Happened at Midnight (Hardy Boys, Book 10)
• The Sinister Signpost (Hardy Boys #15)
• The Great Airport Mystery (Hardy Boys, Book 9)
• A Figure in Hiding (The Hardy Boys #16)
• The Mystery of Cabin Island (Hardy Boys, Book 8)
• The Secret Warning (The Hardy Boys, No. 17)
• The Disappearing Floor (Hardy Boys)