He reports this contact to his commanding officer, Captain Whitney, who also wants Geoffrey to go to Germany, but on a secret mission to steal the plans of the LZ36, a new type of Zeppelin under development at Friedrichshafen.

Geoffrey pretends to be a deserter and travels to Germany, even getting shot in the arm by a British sailor to sell the Germans the ruse.

At Friedrichshafen, he meets his long-time friend Professor Altschul (Marius Goring), who lives with his beautiful and much younger wife, Erika (Elke Sommer), both of whom are scientists working on the new airship.

As soon as it is declared airworthy, to maintain absolute secrecy without first returning to base, the airship is to take part in a military operation to steal or destroy British historical documents, including the copy of Magna Carta, from the National Archives at Balcoven Castle in Scotland.

Geoffrey is to play a key role in the mission, using his knowledge of the Scottish countryside to guide the airship very close to the castle at night while the craft is gliding with its engines off.

After leaving Germany, the zeppelin lands on a fjord in Norway to refuel and take on board a specially trained unit of soldiers armed with mustard gas.

Geoffrey manages to knock him out, and then throws him out an open window, telling the captain the wireless operator had fallen while scraping ice off the airship, as many craft personnel had been ordered to do.

The explanation is believed, but Geoffrey does not have a chance to send any more information because Erika comes on the scene and removes an important radio component.

Meanwhile, alerted by the farmer's report and the radio operator's chaotic call, the British Admiralty scrambles several Royal Naval Air Service squadrons and dispatches ground troops.

Despite desperately lightening the airship in an effort to stay in the air, the survivors are forced to crash-land near the coast of the neutral Netherlands.

[4] The air combat scenes were filmed using Lynn Garrison's collection of World War I replica aircraft, originally assembled for 20th Century Fox's The Blue Max (1966).

During the aerial filming, on 18 August 1970 one of the S.E.5a replicas flown by Irish Air Corps pilot Jim Liddy, collided with the Alouette helicopter used as a camera platform.

"[1] In his review, A. H. Weiler of The New York Times wrote, "... the storied, giant, silver, cigar-shaped dirigible is carrying a flimsy, lighter-than-air spy tale that wouldn't burden a carrier pigeon.