Plot summary (original edition)

In the original version of this story on the way to the new Bayport Airport the boys are almost run over by Mail Pilot Giles Ducroy.Ducroy blames the boys because he lost his job and later frames the boys for a mail robbery.The boys are bailed out of jail by Mr Applegate from Tower Treasure and Mr Jefferson owner of Cabin Island(previous book).the boys don't have any alibi they overhear Ducroy and two other hoods planning a major air mail robbery. They find out that Ducroy and company buy a airplane and stow in the back of the plane to follow the hoods. When Ducroy and company try to steal a major payroll they capture the thieves and the real pilot his plane ruined by the thieves ends up using the crooks plane to deliver his cargo.The boys graduate from high school. They get rewarded since info they had sent to their father clears the boys.

Plot summary (revised edition)

While driving home the Hardy boys take a shortcut which results in a strange car crash and an encounter with an unfriendly stranger. The next day they are hired by Mr. Allen, the president of Stanwide Mining Equipment Company, under the guise of being factory messengers when in fact they are working undercover to investigate missing shipments of expensive mining equipment containing platinum.

The Hardy boys decide to quit their messenger jobs when it becomes clear that the gang is onto them. Instead they rent a plane and take aerial photographs of the area where their car crash occurred, however, their camera and exposed film are stolen before they have a chance to develop the pictures. Continuing to investigate the theft of their camera, as well as the theft of the mining equipment, they repeatedly see footprints and hear voices belonging to Clint Hill, an aircraft pilot who is presumed dead after his plane crashed into the ocean, causing them to wonder if his ghost is somehow involved in the thefts.

After recovering their pictures the Hardy boys rent a helicopter and return to the area of the car crash, finding a large hidden cave and recovering the stolen shipments of equipment. Next the Hardy boys fly to a small Caribbean island, and then to Montana in pursuit of the criminals. Eventually they manage to apprehend all of the criminals involved and return to Bayport gratified at solving yet another mystery.

Original edition (1930)

• Ghostwriter: Leslie McFarlane
• Summary: The boys are framed for air-mail theft and must solve the case to clear their names.

Revised edition (1965)

• Revision type: Completely rewritten; new plot involves the theft of valuable electronics and the "ghost" of a pilot presumed killed in the Caribbean.
• Ghostwriter: Tom Mulvey