The Lady Vanishes (1938, UK)

In Alfred Hitchcock's early, pre-War classic mystery-thriller - and his last British film:

• the opening sequence: due to a snow avalanche, in the Gasthof Petrus inn in the central European country of Bandrika that was besieged by stranded travelers, including English tourist and spoiled socialite heiress Iris Henderson (Margaret Lockwood); she was enroute back to Britain to be married to an aristocrat named Charles; she was soon to meet folk-song musicologist Gilbert Redman (Michael Redgrave) in the room above, who kept her awake by re-enacting a noisy Balkan wedding dance
• the scene of the abrupt strangling (a pair of silhouetted hands signaled the death) of a harmless serenader with a guitar (performing a haunting melody) outside the inn - listened to by one of the hotel guests - the elderly, eccentric ex-governess, spinster, and music teacher Miss Froy (Dame May Whitty)
• the following morning as the train was about to depart, the scene of Iris being hit on the head with a planter box dropped from above, and soon suffering dizziness and hallucinations, while the intended target was actually Miss Froy who was next to her
• during the train ride, after Iris fell asleep with Miss Froy sitting across from her - she then suddenly and mysteriously disappeared - and no one seemed to remember her ("There has been no English lady here") - was there a cover-up?; after her disappearance, a frantic search commenced to locate her; Iris' search was described thusly: "She's back there kicking up a devil of a fuss that she's lost her friend"
• one of the clues - the sight of Miss Froy's handwriting on the train window, from earlier when she was having tea with Iris; because of a loud train whistle noise, she wrote her name with her finger on the frosted glass: ("FROY - it rhymes with Joy!")
• another clue - 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
• the eerie appearance of a fully-bandaged or mummified "patient" - allegedly brain surgeon Dr. Hartz's (Paul Lukas) patient, but revealed to be a disguised and/or kidnapped Miss Froy
• the discovery that Dr. Hartz's suspicious, working-class, supposedly mute British nurse-nun attendant (Catherine Lacey) was wearing high heeled shoes - was she one of the foreign agent conspirators who was involved in Miss Froy's abduction?
• after drugging Iris and Gilbert with doctored drinks, Dr. Hartz admitted to them that he was involved in the conspiracy: "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 didn't comply with his request to drug them, and they faked the drug's effect

Revelations
Bandaged "Patient" - The Kidnapped Miss Froy?	Nurse Wearing High-Heel Shoes
Conspirator Dr. Hartz	Unwrapping Miss Froy's Bandages	Miss Froy's Confession - About Being a Spy

• Iris and Gilbert freed bandaged Miss Froy (who was replaced by one of the conspirators); there was a resultant gunfight shootout with soldiers from the Morsken station after the train was diverted onto a branch line and stopped
• the scene of Miss Froy's confession to Iris and Gilbert of her secret mission to deliver a musical coded message - "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. l 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).]
• the very last frames of the film set in the Foreign Office in London -- a joyful reunion between Iris, Gilbert (who had forgotten the memorized tune of the all-important folk tune), and Miss Froy; the couple heard the tune being played in an adjoining room - and found Miss Froy seated at a piano playing the cryptic melody (containing a coded secret message)

Reunited with Miss Froy, Seated at Piano Playing Tune in the Foreign Office in London