After visiting the fictional country of Bandrika, English tourist Iris Henderson is returning home to get married, but an avalanche blocks the railway line. The stranded passengers are forced to spend the night at a hotel. In the same predicament are Charters and Caldicott, cricket enthusiasts anxious to see the last days of a Test match in Manchester, and Miss Froy, Selina Cadell, a governess and music teacher.
That evening, Miss Froy listens to a street folk singer, but he is strangled to death by an unseen murderer. Iris is annoyed by a loud noise from the room above hers, caused by Gilbert Redman, an ethnomusicologist who plays the clarinet for local folk dancers. She tries to have him removed, but he confronts her and she fails. The next morning at the railway station, Miss Froy drops her glasses near Iris and her friends. When Iris tries to return Miss Froy’s glasses, she is hit on the head by a large planter dropped from above. Miss Froy helps her onto the train. As Iris waves goodbye to her friends, she faints and then comes to in a compartment with Miss Froy and several strangers. Also on board are Charters and Caldicott, Gilbert, and Eric Todhunter with his mistress, who is passing herself off as "Mrs Todhunter."
Iris joins Miss Froy in the dining car for tea. Miss Froy gives a package of her favorite tea to the waiter and says she only drinks this brand. Soon after they return to their compartment, Iris falls asleep. When she wakes up, Miss Froy has vanished. The other passengers in the compartment, several other passengers and train staff deny ever seeing her. Todhunter pretends not to remember her to avoid drawing attention to his liaison with his mistress. Julian Rhind-Tutt as Mr Todhunter and Keeley Hawes as Mrs Todhunter are part of the tense dynamics, while Gilbert agrees to help Iris search for Miss Froy.
Angry at her lover for his reluctance to pursue a divorce, Todhunter’s mistress admits to seeing Miss Froy. At the first stop, Dr Hartz’s patient, whose head is completely bandaged, is brought aboard on a stretcher and accompanied by a seemingly deaf-mute nun. A woman dressed exactly like Miss Froy, Madame Kummer, appears in her place and claims to have helped Iris after she was struck on the head. After Todhunter tells his mistress his wife would never agree to a divorce, she says that Madame Kummer was the woman she saw. Iris briefly doubts her memory, but while in the dining car with Gilbert, she recalls having tea with Miss Froy and sees where the woman wrote her name on the window. She pulls the emergency brake and faints again.
Gilbert still believes Iris is hallucinating, but when one of the cooks tosses trash out the window, he sees a tea package wrapper briefly stick to a corridor window and recognizes the special brand Iris told him Miss Froy drank, convincing him of her existence. While continuing their search, Iris and Gilbert find Miss Froy’s glasses in the baggage car. However, they are attacked by Signor Doppo, who was in Iris’s compartment. They lose the glasses in the struggle, and Doppo escapes.
Iris notices that the nun accompanying Dr Hartz’s patient is wearing high heels, and they suspect that the patient has been replaced by Miss Froy. Dr Hartz takes them to the dining cars for a drink and accompanies them back to a compartment, where he admits that the patient is Miss Froy, that he is involved in the conspiracy and that they have been drugged. However the false nun, revealed to be a British woman in costume, did not put the drug into their drinks out of loyalty to her fellow countrymen. Iris and Gilbert escape, free Miss Froy, and replace her with Madame Kummer.
When the train stops near the border, Dr Hartz discovers the switch. He has part of the train diverted onto a branch line, where soldiers wait. Gilbert and Iris inform their fellow passengers what is happening. A uniformed soldier boards and requests that they all accompany him. They knock him out and take his pistol. Another soldier fires, wounding Charters in the hand.
During the ensuing gunfight, Miss Froy tells Gilbert and Iris that she must get away as she is actually a British spy. Just in case, she gives them a message (encoded in a tune) to deliver to the Foreign Office in Whitehall — the same tune that the murdered street musician performed for her. Miss Froy then runs into the forest, and those on the train don’t know whether or not she is shot. Todhunter attempts to surrender, waving a white handkerchief, but is shot dead. Gilbert and Caldicott commandeer the locomotive, but the knocked-out soldier wakes up and threatens the rest of the guests. The false nun escapes through a side door and switches the tracks but is shot in the leg. However, Gilbert and Caldicott manage to pull her up into the train before she is left behind.
In London, Charters and Caldicott discover the Test Match has been canceled due to flooding. Seeing her fiancé from a distance at the station, Iris jumps into a cab with Gilbert, who kisses her. They arrive at the Foreign Office, but in the waiting room Gilbert realizes he cannot remember the vital tune. As they are led into the office, Gilbert and Iris hear it. The doors open, revealing Miss Froy playing the tune on a piano.
