Traveling aboard a transcontinental train, young Iris Henderson (Margaret Lockwood) becomes alarmed when an acquaintance, elderly governess Miss Froy (Dame Mae Whitty), suddenly vanishes. Inexplicably, all the other passengers deny having seen the woman. So Iris turns to her lone ally -- handsome music scholar Gilbert Redman (Michael Redgrave) -- for help. As the two search for clues to Froy's disappearance, they uncloak a sinister plot.