She could have danced all night, she being Marchesa Marcella Galdi (Greta Nissen), an Italian noblewoman who has enjoyed a romance with Austrian military man Captain von Homberg (Karl Ludwig Diehl), and this evening they have been lost in each other's arms on the ballroom floor, a fact noted by some of the Captain's fellow officers with some disdain, since Italy and Austria are currently in a state of diplomatic tension. But there’s more to it than that, as Marcella is actually a spy for the Italian government who never intended to fall in love with one of the other side, and when her secret work places him in danger she did not intend thanks to her scheming superiors, she is full of remorse, especially when this means they may be torn apart forever...
On Secret Service, also known as Spy 77 after the agent the characters are trying to track down, was a remake of a German espionage effort, as was the fashion in Europe once sound had been introduced and the silents were on the way out. The trouble with that was, obviously, not everyone spoke ever language in Europe, so there was a lot of swift remaking of various properties in the local lingo, indeed some films were shot simultaneously in different languages with different casts to get the studios' money's worth out of their material. This example showed up just as events were beginning to get... interesting on the Continent, shall we say, as the mood darkened politically and there was good reason for cinema talent to emigrate from, for instance, Nazi Germany.
Nissen was Norwegian, a good friend of the Royal family there, so was very highly thought of, but her accent was her undoing when sound was introduced: she had enjoyed a promising silent career, but unlike Greta Garbo audiences never took to her intonation and before the nineteen-thirties were over, so was that career. One of the nearly-women of the era, she did secure work in a few titles, and this was unusual in that it featured a cast led by non-English natives for a film that had been made for the British and English-speaking market, as her romantic lead Diehl was a German star who made a few moves to international success, but then allied himself with the Nazi film industry and did very well during the war essaying authority figures. Naturally, when the conflict ended, it was clear he had well and truly backed the wrong horse, and his subsequent opportunities reflected that.
Back at the plot, it is likely On Secret Service was going to be more of historical interest than appeal to the general audience in the next century, though it was intriguing on its own terms despite it being a rather dry tale of derring-do which only really sprang into life when aeroplanes were involved. There was a good reason for that, director Arthur B. Woods was a keen aviator, and you could tell these scenes with the aircraft sparked his enthusiasm (he had a habit of flying the plane stunts in his films himself), though his passion would be his undoing as he signed up for the Air Force in the Second World War and was brought down in the process, the only British director to be killed in the air. Again, the background here was more engaging than what played out on the screen, though Nissen evidently appreciated all those frocks she got to wear (gold lame floor length gown, dalmatian-spotted backless number, and so on), but you couldn't discern the supposedly heated love affair between her and Diehl's characters was anything but a narrative convenience, until the unintentionally amusing finale, anyway. There was a fun birdsong music box that passed on Morse code messages, however.
[Network have restored and rereleased this on Blu-ray and DVD. There's an image gallery as an extra.]