On a train headed for England a group of travelers is delayed by an avalanche. Holed up in a hotel in a fictional European country, young Iris (Margaret Lockwood) befriends elderly Miss Froy (Dame May Whitty). When the train resumes, Iris suffers a bout of unconsciousness and wakes to find the old woman has disappeared. The other passengers ominously deny Miss Froy ever existed, so Iris begins to investigate with another traveler (Michael Redgrave) and, as the pair sleuth, romantic sparks fly.