Ireland, 1944. Bridie Quilty is a beautiful, spirited, but not terribly bright young Irish woman. After growing up listening to her father's almost certainly untrue stories of The Irish Revolution, Bridie has developed an abiding hatred for English people. When she turns 21 she goes off to Dublin and attempts to join the IRA, but the old IRA leader she approaches, now content with an independent Ireland, blows her off.
However, Mr. Miller, a man she meets on the train, takes her more seriously. Miller is actually a Nazi spy, who is headed to England with the intention of breaking another Nazi spy, Oscar Pryce, out of jail. Recognizing Bridie's potential as a Honey Pot, he recruits her into becoming a German agent. They travel to the British village of Wynvale, near where Pryce is being held, and Bridie uses her good looks to worm information out of guards at the prison. When a British intelligence agent, Lieutenant David Baynes (Howard) shows up in Wynvale, Miller orders Bridie to flirt with him in order to distract him from the prison break.
Using Bridie's info, Miller breaks Pryce out—but the British send out patrols, Pryce is shot dead, and Miller is mortally wounded. It's Bridie herself who winds up going off to the Isle of Man to retrieve Pryce's intelligence. Following after her is David, who's fallen in love with her. He has to figure out how to stop the Nazi plot and stop Bridie from going to prison for espionage.
- Bedsheet Ladder: How David helps Bridie to escape. He fashions a rope out bedsheets so she can get away before the imminent arrival of the Border patrol people trying to arrest hill.
- Captain Ersatz: The idiot security officer on the Isle of Man and his smarter sidekick were supposed to be Charters and Caldicott, a popular Those Two Guys comic pair played by Basil Radford and Naunton Wayne. After Radford and Wayne held out for bigger parts in the movie, their characters were placed by the suspiciously similar Captain Goodhusband and Lieutenant Spanswick.
- Catapult Nightmare: How Bridie wakes up after a nightmare about having to dispose of Miller's body.
- Deus ex Machina: How do Bridie and David escape from the Germans? It turns out that the funeral possession that the Nazis were trailing behind on the road turned out to be a caravan smuggling illicit alarm clocks (???) over the border. In the chaos after the whole gang of clock smugglers runs from Border Patrol, Bridie and David make a break for it and run away from the Nazis.
- Disposing of a Body: After Miller dies in her room, Bridie smuggles the body out in plain sight of everyone by putting him in a wheelchair and dressing him up to look like the wheelchair-bound old man who lives in the inn.
- Hey, Wait!: Bridie is visiting the parliament building on the Isle of Man. She's about to leave the gallery when someone says "Stay where you are!" It's a photographer trying to snap a picture.
- Hollywood Darkness: It is strangely well-lit at 9 pm on the hill where Bridie and David are sitting, which is even stranger because it's pitch-dark at the railway station where Miller is sneaking Pryce into a car.
- Honey Pot: Bridie is a mild version of this, in that she uses her good looks to get soldiers talking, which allows her to pass info about when and how Pryce is being moved.
- How We Got Here: After an opening shot of David racing around the dark streets of the Isle of Man, the narration says "We've started this tale at the wrong moment." Then it cuts back to the beginning of the story with Bridie in her Irish village.
- Inner Monologue: Bridie's inner monologue is heard several times, like when she's sizing up Miller on the train, or when she's considering the notebook and finally figuring out that the secret intelligence must have something to do with the impending invasion of France.
- MacGuffin: The secret info contained in Pryce's notebook, namely, intelligence regarding the location of the D-Day landings.
- Mathematician's Answer: Miller is bleeding to death after he was shot by a British patrol, but he's still got enough life left to snark at Bridie.Miller: There's a bullet in me.
Bridie: How do you know?
Miller: Because it didn't come out.
- Miles Gloriosus: Although not directly stated it's very strongly implied that Bridie's father Danny is completely fabricating his stories of heroism with the IRA. When Bridie visits her father's supposed comrade-in-arms Michael Callaghan, a real IRA veteran, Callaghan has no idea what she's talking about.
- Not What I Signed Up For: The utterly clueless Bridie, who was captivated as a girl by her father's stories of fighting the English, becomes a goddamn Nazi spy out of some sense of grand adventure. When she retrieves the notebook and realizes that it's intel about the D-Day landings and Allied soldiers (including Irish volunteers) will die if she hands it over, she burns the notebook.
- Reveal Shot: A camera shot shows the railway station operator telling the cops that nobody suspicious has been in the station that night. The camera then pans left to reveal that the Nazis are in the room behind the station operator, and one has a gun jammed into the station operator's back.
- Sexy Secretary: Not only does Capt. Goodhusband's hot young secretary wear a very tight ATS uniform, she makes a point of hopping up onto Goodhusband's desk and dangling her bare calves when taking a letter.
- Trust Password: How German agents recognize each other. Miller and his contact recognize each other in a bookstore when Miller says "There's nothing like books for collecting dust", and his contact says he's buying a biography of Lord Curzon. Bridie's attempt at this is short-circuited when, just after she gives the sign on the train (asking for the window to be half-open), the old lady who is her German contact is arrested before she can give the countersign (telling Bridie it can only be open for 10 minutes).