The novel opens with the marriage of Sybil Berners, a wealthy and passionate heiress from Virginia, and John Lyon Howe, a brilliant but impoverished lawyer who takes the name Berners. Their early days are consumed by intense mutual devotion, yet a shadow of impending doom hangs over Sybil. During their travels, they encounter a stranger named Rosa Blondelle, whose husband has cruelly robbed and deserted her. Moved by romantic generosity, Sybil invites Rosa and her child to live at Black Hall, their ancient manor in the Black Valley.
As life at Black Hall resumes, a subtle and destructive tension develops. John Lyon grows increasingly captivated by Rosa's beauty and musical talents, spending long hours in her company. Sybil, possessing the fiery and jealous nature characteristic of her family, suffers in silence, concealing her agony behind a mask of outward calm. Her inner turmoil grows into a consuming obsession as she watches her husband drift toward their guest.
To break the domestic monotony and test her household, Sybil organizes a lavish masquerade ball for All-Hallow Eve. The festive night brings together the gentry of the region in elaborate costumes and masks. Beneath the glittering surface, jealousy and suspicion reach a fever pitch. Sybil exchanges her costume with a friend to observe the guests unsuspected, witnessing her husband's continued devotion to Rosa.
The night culminates in sudden violence when Rosa is found stabbed to death in her chamber. With her dying breath, she accuses Sybil of the murder. Facing overwhelming circumstantial evidence and a charge from her dying rival, Sybil finds herself isolated by suspicion. Refusing to succumb passively, she navigates the chaos of accusation and flight, as the dark secrets of the manor come to light.
