Illegitimate Diantha Fraser considers herself fortunate to have found employment as a music teacher at a fashionable ladies' seminary. Her mundane routine is shattered when summoned to her father's deathbed. Shortly thereafter the Earl of Devonwych dies and much to her astonishment bequeaths Diantha a modest fortune with strings attached. She will lose most of her inheritance unless she marries his protege, Sebastian, Marquess of Ravistock.

Sebastian is equally jarred by the terms of his mentor's will. His lineage demands he look higher for a wife than Devonwych's by-blow. Yet he feels honor-bound to keep his promise to the earl, to protect Diantha from her half brother Basil's wrath.

Rising to the challenge, the marquess solemnly swears to foil all of Basil's greedy schemes. Yet, while preoccupied with playing her knight-errant, Sebastian somehow loses his heart to an equally smitten Diantha.