Separated since youth following her mother's passing, Madeline Kensington returns from the countryside to fulfill an arranged matrimonial promise with Sebastian Prescott. But the boy who once gave her a keepsake bracelet has forgotten his pledge. Viewing his unassuming bride as a greedy opportunist, Sebastian treats her with open condescension, hopelessly obsessed instead with a mysterious masked operative known as Onyx who once saved his life.
Madeline conceals the startling reality that she is the very vigilante her aloof spouse idolizes. When she realizes Sebastian leads his own covert vigilante operations, she prepares to disclose her alter ego, only to overhear him disparaging her character to mockery-filled socialite peers. Stung by his bitter contempt, Madeline decides to sign the divorce papers, forcing her arrogant husband toward a shattering reckoning when the truth inevitably surfaces.
