The film begins with a young schoolboy, Michael Walton (Toby Regbo) asking about his great-aunt Anne Keys. Anne grew up the adoptive daughter of Sir Alexander Keyes, a notable politician in favour of appeasing Nazi Germany. Anne lives a charmed life, spending most of her days running round the family ruins in Gorgeous Period Dress and flirting with her boyfriend, Lawrence. Until one day, she organises a birthday party for her father... and the mysterious Mr Balcombe shows up.
At the party, family friend Hector Haldane delivers a passionate speech about the need to stop Nazi tyranny — and two weeks later apparently kills himself. Tragic — but hidden in the family archives Anne finds a recording of Hector, which makes it clear that someone was blackmailing him.
Meanwhile, at a family picnic the baby Anne was supposed to be looking after goes missing, and only after Anne has roused the entire family to search the woods is he found — safe and sound, in a place she'd already looked. Anne knows Balcombe has something to do with all of this, but now everyone else thinks she's hysterical.
Or they appear to, anyway. Most of the recordings in the archives have been taken away by Balcombe, but Anne has kept a few hidden. Desperate to find some proof of a conspiracy, she listens to them all and hears her brother's voice come through loud and clear. Politicians at the highest level of government have been involved in a plan to avoid war at all costs — and now want peace at any cost — and Anne's family are at the heart of it.
- Anyone Can Die: And does.
- Aristocrats Are Evil: The point is heavily made that it's the privilege and the blindness that comes with that that causes the action in the film. The three most sympathetic characters - Anne, Hector and Gilbert - are not aristocratic by birth.
- The Atoner: Walter is clearly saddened for the part he played in discrediting Anne.
- Awful Truth: The family is involved in a treasonous plot to make peace with Hitler.
- Blue Blood: A major theme — Anne is adopted, remember. And was the child of gypsies, as it happens — this is the implicit motive behind her family's attempts to make peace with Hitler. Because if they're not at war with Germany, Germany can't win...
- Break the Cutie: Anne is the victim of gaslighting, her family are completely corrupt, her boyfriend is murdered, two of her friends apparently commit suicide, and then her family lock her in a small room and sedate her, possibly for years.
- Bring News Back: Once Sir Alexander is proved corrupt, Anne tries to send the evidence to Winston Churchill... and fails.
- Dead Star Walking: David Tennant.
- Disposing of a Body: You can hang him from the rafters with the other dead animals!
- Does This Remind You of Anything?: At one point in the film, someone notes that the declaration of war means that people are having their pets put down by the thousands. Later, while running from the vet's, Anne comes across people throwing the dead animals onto bonfires.
- Doomed Moral Victor: Hector. Not to mention Gilbert, Lawrence and Anne, though Anne at least survives the film.
- Driven to Suicide: Hector, probably. It's only ambiguous because Balcombe is perfectly capable of faking it.
- Fallen Princess: Anne.
- Foregone Conclusion: We know from the beginning that Anne is going to disappear. But as it happens, she gets away from her family — she just drops off the radar afterwards, until the end.
- For Your Own Good: Anne has Romany heritage and her family are desperate to stop Hitler from invading Britain — so they want to make peace.
- Framing Device: The story begins with Michael Walton, a present-day schoolboy, asking his elderly cousins what happened to his great-aunt Anne, who disappeared during the war.
- Gaslighting: Balcombe is doing this to Anne in order to destroy her credibility. Her family are helping him.
- Head-in-the-Sand Management: Sir Alexander.
- Idiot Ball: But why doesn't Anne send the evidence to her political heavyweight father, whom she loves and trusts absolutely? Subverted, however. In the event, it's just as well she didn't, though it doesn't do her any good.
- Madwoman in the Attic: Her family attempt to turn Anne into one of these.
- Narrowed It Down to the Guy I Recognize: Played with. In a cast of heavyweights, every other person must have something more to do than that bit part — right? Ultimately, Bill Nighy is not merely a supportive but curiously ineffective helper to Anne.
- Noble Fugitive: Anne, until the end.
- Noodle Incident: We aren't told what Balcombe was blackmailing Hector with, only that "people do all sorts of things when they're young."
- Oh, Crap!: When Anne hears Ralph on the meeting recording
- Pet the Dog: Anne's mother unlocks the door to let Anne escape. However, this is subverted when it turns out to be a final ploy to get Anne to go along with the family
- Precision F-Strike: When Anne gets confirmation that her family has indeed been drugging her to keep her from talking about the peace deal her response is "You FUCKING bastards, you have nothing to do with me!," followed by a Cluster F-Bomb.
- Reasonable Authority Figure: Sir Alexander. Apparently, anyway. Poor Anne. Churchill, although never appearing, is also treated as this.
- Sacrificial Lamb: Hector.
- Shout-Out (with a touch of Literary Allusion Title): The title refers to the year of the events, but very likely also has something to do with another wartime conspiracy thriller with 39 in the title. Since the film aspires to a Hitchcockian aesthetic and feel the title is likely more than just a coincidence, though interestingly the emphasis in the plot on the motivating factors behind the war resonates more with Buchan's book than Hitchcock's film.
- Stiff Upper Lip: While it's a very stressful time for all the characters and one can't blame Anne in particular for getting quite a bit upset, it's notable that the declaration of war occurs while Anne and Gilbert are filming a scene. The director stops them, announces the news (as it's important and he felt everyone should know as soon as possible), and then they go back to work.
- Upper-Class Twit: Equally, no one in this film is at all stupid. Unfortunately for Anne.
- Video Wills: Gilbert leaves a message for Anne in the scene they film together; she only realises it when she's called back for redubbing.
- "Where Are They Now?" Epilogue: In the frame story.
- You Have to Believe Me!: Often averted. Most of the people Anne appeals to about the conspiracy believe her after explanation and a look at the records - except her father, who is clearly humouring her. Used, however, in the scene where baby Oliver goes missing.