Plot summary (original edition)

The original story line involves the "Flying Express" railway. This railway passes close to a foreign spy camp hidden in a forest.

Plot summary (revised edition)

After the new hydrofoil they are guarding, the Flying Express, is stolen, the Hardy Boys face frequent danger in solving a mystery involving criminals who operate by signs of the zodiac.

Original edition (1941)

• Ghostwriter: John Button
• Summary: The boys foil a foreign spy training operation using the "Flying Express" train.

Revised edition (1970)

• Revision type: Completely rewritten, with no plot elements retained from the original; the new story involves the disappearance of the "Flying Express" hydrofoil.
• Ghostwriter: Vincent Buranelli