The morning after Julia Ross (Nina Foch) takes a job in London as secretary to wealthy widow Mrs. Williamson Hughes (Dame May Whitty), she wakes up in a windswept Cornish mansion, having been drugged. Mrs. Hughes and her volatile son, Ralph (George Macready), attempt to gaslight Julia into believing she is Ralph's wife, Marion. Her belongings have been destroyed, the windows barred and the locals believe that she is mad. Will Julia be able to escape before she falls prey to the Hughes sinister charade? And what happened to the real Marion Hughes?