The Lady Vanishes (1938, UK)

In Alfred Hitchcock's early, pre-War classic mystery-thriller - it was his last British film before the director moved to Hollywood, and based upon Ethel Lina White's 1936 novel "The Wheel Spins":

• in the opening sequence, due to a snow avalanche, the small Gasthof Petrus village inn in the central European country of Bandrika was besieged and overcrowded by stranded travelers from the train, including English tourist and spoiled, engaged socialite heiress-playgirl Iris Matilda Henderson (Margaret Lockwood); she was enroute back to Britain to be married to an aristocrat named Charles; she was soon to meet folk-song, ethno-musicologist Gilbert Redman (Michael Redgrave) in the room above, who kept Iris awake by re-enacting a noisy Balkan wedding dance
• in a startling scene outside the inn, a harmless serenader with a guitar (Roy Russell), who was performing a haunting melody, was abruptly strangled (a pair of silhouetted hands from an unseen killer signaled his death); one of the hotel guests - the elderly, eccentric ex-governess, spinster, and music teacher Miss Froy (Dame May Whitty) strained to listen to him due to the noisy folk-dancers
• the following morning with the tracks cleared, and as the train was about to depart, Iris was hit on the head with a planter box dropped from above; she began to suffer dizziness and hallucinations; she didn't know that the intended target was actually Miss Froy who was next to her; Iris regained consciousness in a compartment where Miss Froy and other non-English-speaking passengers were seated
• during the train ride, Iris shared a cup of tea with the kindly Miss Froy in the dining car (Miss Froy used her own preferred tea bag); because of a loud train whistle and track noise making it difficult to hear, she had written her name with her finger on the frosted glass of the window: ("FROY - it rhymes with Joy!")
• once back in the compartment, Iris fell deeply asleep with Miss Froy sitting across from her - but then the old lady had suddenly and mysteriously disappeared when she awoke - and no one seemed to remember her: ("There has been no English lady here") - was there a cover-up, or where there other reasons for denying her presence?; after her disappearance, a frantic search commenced to locate her, with Iris assisted by Gilbert; Iris' search was described thusly: "She's back there kicking up a devil of a fuss that she's lost her friend"; brain surgeon Dr. Egon Hartz (Paul Lukas) excused Iris' account of the absent Miss Froy by blaming it on delusionary hallucinations following her head-concussion
• one of the clues Miss Froy had left behind was her handwriting on the train window; shortly later, a second clue was discovered - the momentary sight of a Harriman's Herbal Tea packet-bag (Miss Froy's own personal favorite tea brand) stuck to the train window pane after the garbage was tossed through a window by the train's cook; even later, Miss Froy's broken glasses were found in the baggage car
• at the train's first stop (where no one exited), an eerie-looking, fully-bandaged or mummified brain surgery "patient" - allegedly Dr. Hartz's patient, was brought on-board on a stretcher; the patient was accompanied by Dr. Hartz's suspicious, working-class, mute British nurse-nun attendant (Catherine Lacey); afterwards, a Miss Froy look-alike, named Madame Kummer (Josephine Wilson), mysteriously appeared on the train, and claimed to have accompanied Iris after her head concussion; it was thought that Miss Froy ("The Lady") had Vanished
• Iris became suspicious when she noticed that Dr. Hartz' attendant was wearing high heeled shoes - was she one of the foreign agent conspirators who was involved in Miss Froy's abduction?; Iris suspected a conspiracy and worried that the bandaged patient (Madame Kummar), who had gained entry into the train in disguise, might be switched with Miss Froy, in order to have them swap places; Miss Froy would be kidnapped (and removed from the train in bandages), and then taken to a hospital and murdered during an alleged operation
• after drugging Iris and Gilbert with doctored drinks, Dr. Hartz admitted to them that he was involved in the conspiracy to remove Miss Froy from the train, disguised as his patient: "l am in this conspiracy as you term it. You are a very alert young couple, but it's quite useless for you to think of a way out of your dilemma. The drink you've had now, l regret to say, contained a quantity of Hydrocin. For your benefit, Hydrocin is a very little known drug which has the effect in a small quantity of paralyzing the brain and rendering the victim unconscious for a considerable period"; however, Hartz didn't know that the Nurse (a British woman who felt loyalty to England) didn't comply with his request to drug them, and they faked the drug's effect

Revelations
Bandaged "Patient" - The Kidnapped Miss Froy?
	Dr. Hartz' Nurse-Nun Attendant Wearing High-Heeled Shoes
Conspirator Dr. Hartz (Paul Lukas)	Unwrapping Miss Froy's Bandages	Miss Froy's Confession - About Being a Spy

• Iris and Gilbert located the bandaged Miss Froy (who was being prepared to be removed from the train at the next stop), unwrapped her bandages, and replaced her with Madame Kummer - against her will; at the next train stop, Dr. Hartz and his "patient" (Madame Kummer) departed from the train to enter an ambulance, but once Dr. Hartz realized that Miss Froy had been freed and swapped with Madame Kummer, he ordered the train to be diverted onto a branch line and stopped
• there was a resulting gunfight shootout with uniformed military soldiers from the Morsken station who boarded the train; during the chaos, Miss Froy admitted to Iris and Gilbert that she was a British spy, and that her secret mission was to deliver a musical coded message; she explained that she would share her encoded message with them, just in case: "In case I'm unlucky and you get through, I want you to take back a message for a Mr. Callendar at the Foreign Office at Whitehall....It's a tune. It contains, in code of course, the vital clause of a secret pact between two European countries. I want you to memorize it" - [Note: Miss Froy was a British spy in disguise, who had memorized the film's MacGuffin (a musically-coded state secret hidden in folk music). It was the same tune that was performed by the assassinated guitarist at the inn.]; Gilbert boasted that he could easily memorize the tune; after transferring the code to them, Miss Froy fled from the train into a nearby forest
• by the film's conclusion set back in London, Iris and Gilbert had apparently fallen in love; the two took a taxi-cab to the Foreign Office in London, and were taken to a waiting room where Gilbert sheepishly admitted that he had forgotten the memorized tune of the all-important folk tune
• in the very last frames of the film, the couple heard the tune being played in an adjoining room, and were joyfully reunited with Miss Froy; she was seen seated at a piano playing the cryptic melody (containing the coded secret message)