The Michael York character in this 1971 movie is no James Bond type, as seen in "A View to a Kill" (1985), where airships were also key to the plot, yet it is clear that he was being forced to choose between the two major warring European powers during the First World War as regards his loyalties.The plot centres around what appears to be a "simple" task for one Lt. Geoffrey Richter-Douglas, a man with German roots yet serving in a minor role in the British Army: get back to Germany and find out all the technical details of a new Navy Zeppelin called the LZ36, the latest in a series of airships used for the strategic bombing of British cities in 1915 in order to terrify the civilian population.
Having been deliberately shot in the arm (literally) in order to convince the Germans that he was genuinely turning his back on the British, Richter-Douglas finds that the purpose of the new airship's sole mission is much more sinister than that - no bombs are on board, for a start. This mission outrages the airship's designer, Professor Altschul (Marius Goring) and worries his wife, Erika (Elke Sommer). Erika keeps her feelings for Richter-Douglas carefully neutral, and her contention that he is actually still working for the British to herself.
The Scotsman is nearly discovered, but he despatches one crewmember in a particularly brutal fight and dumps him over the side while the Zeppelin is still in flight; fortunately for him, the ruse that he was trying to save him fooled the commander of the ship (Andrew Keir) and the army colonel (Anton Diffring) leading the mission.
The mission (which I won't reveal here) ultimately fails, but without his help, though Erika's presence appears to add little to the plot except provide a distraction to the otherwise male-dominated scenario, as military missions in those days tended to be. The most exciting part is perhaps the aerial battle where British fighters try and put as many bullet holes into the airship as possible before the ship climbs out of reach when many things (including the bodies of killed crewmembers) are themselves dumped over the side.
Nonetheless, the ship is heading for a "Hindenburg"-style fiery death as (for some reason never explained) the ship loses height and crashes into the North Sea by the Dutch coast. Perhaps predictably, both Richter-Douglas and Erika are the only survivors of what was a doomed mission (from the British point of view, it could only be doomed).
As espionage stories go, this one would have been better had the action not been so plodding at times: one might even say that the mid-North Sea fuel stop was a rest-stop for the movie itself. It is really only from when the enemy airship reaches its destination that the action really does get going. For fans of airships past and present, it is memorable for the fact that the set used for the inside of the ship was remarkably detailed and that the genuine airship sheds at Cardington, which still exist, were used as a backdrop.
Overall, however, it is a good film to watch because, unlike others, it focuses mostly on the troubled psyche of the man, who is clearly being made a pawn by both sides to get what they want, and does not attempt to glorify war. The subtle anti-war message comes when Erika asks him to comment on the failure of the mission, and he says bluntly that he "didn't want any of it". For him, survival was his top priority - what anyone else wanted did not matter to him, even if he never had to take a stand against either side.