Iris Henderson (Margaret Lockwood) is returning home by train after celebrating her last days as a single woman. She has befriended a kindly Governess who helped her board the train after an odd incident where she was grazed by a falling flower pot. After dining with the elderly lady and returning together to their compartment, Iris wakes up from a nap to discover Miss Froy has vanished. She is astonished and angry to learn that none of the other passengers can verify her existence. She enlists the help of a young musician, Gilbert (Michael Redgrave), who is more interested in her then the disappearance of Miss Froy