Set during the First World War, a German U-Boat commander is sent on a secret mission to infiltrate the Scottish Orkney Islands to destroy the British naval fleet stationed there, but an English school teacher will soon thwart his plans.
The score was composed by Miklós Rózsa.
The film shows the following tropes:
- Did Not Get the Girl: Hardt makes it obvious that he's very interested in Tiel, but she rejects his advances. It's obvious why when we find out she's playing for the other team, Britain.
- Evil Wears Black: Our eponymous spy is always in black. Not only for symbolic purposes, but also he steals away in the dead of night and black is the best disguise.
- Going Down with the Ship: Captain Hardt decides to go down with the ferry when he finds out that his German U-Boats have fallen for a trap and escaping the sinking ferry would be futile.
- Impersonation Gambit: Fraulein Tiel is actually Jill Blacklock, not the real German operative.
- The Infiltration: Hardt steals away into the night on a smuggled motorcycle to the school and is hidden away by the Fraulein Tiel.
- Nazi Protagonist: Although the film is set during World War One, Hardt (Veidt) is coded as a Nazi. And he's our German protagonist: we see him receive his mission and eventually see him being betrayed by who he thought were loyal Germans.
- Officer and a Gentleman: Hardt is a gentleman especially when he decides to go down with the ferry.
- Out-Gambitted: The Germans planned to kill the school teacher heading to the restricted island and replacing her with a lookalike German. However, the British caught wind of this plan and replaced the German with their own operative to foil the plan.
- The Reveal: Fraulien Tiel is actually British and is married to the disgraced navy officer, Ashington (Shaw). They were both sent to foil the Germans plan to destroy the British fleet.