Isbel Loment, engaged to the ordinary and unexceptional Marshall Stokes, leads a peripatetic existence as the ward of her aunt, Ann Moor. Their travels take them to the downlands of Sussex, to Runhill Court, an ancient home owned by Henry Judge. There Isbel discovers a strange staircase few can see, which leads upwards to three doors. She chooses one, which opens onto a room that appears to exist only part of the time; what might lie behind the other doors remains a mystery. In the room she reencounters Judge. There they find new insights and are able to express themselves in new ways, but are unable to recall what has transpired there when they leave. They develop a disturbing parallel relationship in the mysterious room, which ultimately culminates in the death of Judge and the rupture of Isbel from Marshall.