Glamorous socialite Iris Carr is on her way back to England from a European summer holiday and looking forward to the comforts of home, when she strikes up a conversation with the kindly Miss Froy on the train to Trieste. Irene warms to her companion, and is alarmed when Miss Froy suddenly disappears from the train. Worse still, she is horrified to discover that none of the other passengers on the train will admit to having never seen such a woman. Doubting her sanity and fearing for her life, Iris is determined to find Miss Froy before the train journey is over.