Iris Carr, a young English socialite, is traveling alone by train through pre-World War II Europe. She befriends Miss Froy, an elderly English governess. After a brief nap, Iris wakes to find Miss Froy missing. Strangely, none of the other passengers recall ever seeing her. Determined to find the truth, Iris enlists the help of fellow passenger Gilbert. Together, they uncover a web of espionage, realizing that Miss Froy is actually a British agent carrying a secret message encoded in a musical tune.