In 1797, the shaman of the Dakota tribe of the Black Hills goes off alone to seek a vision from the Great Spirit. What he sees confuses him at first. He sees a sun in the night sky, a herd of buffalo led by a grey eagle, and a white mountain lion with blue eyes. A short time later, he understands the vision when daughter of the tribe's leader returns though she was believed dead. In her arms, she carried a small blond child, entrusted to her care when the child's mother died. This is how Christina came to grow up among the Dakota Indians although she and her family had always known that she would some day be returned to the white man's world.

The Marquess of Lyonwood, known by his friends simply as "Lyon," arrives home just in time to witness the deathbed confession of his wife. Her name is Lettie and she dies trying to give birth to the child of Lyon's brother, James. He removes himself from love, vowing that he'll never again marry. He does, however, continue to care for his family and he's accompanying his sister, Diana, during her first London social season when he meets Christina. Having been called a "lioness" by her own people, Christina's immediate reaction to Lyon is that she's found her mate. Although she plans to marry and deal with the bequests from her grandfather's estate, she has no plans to remain in England. She wants to return to America and to her Indian family. With that plan in mind, she proposes to Lyon, believing that he would make an appropriate husband for her short stay in London.

Lyon refuses the proposal but cannot deny the attraction he feels for Christina. That attraction grows mutually as Lyon exhibits all the attributes of a warrior and traits that Christina has grown up believing to be desirable but that many young London socialites find frightening. Christina is very self-sufficient, which Lyon in turn finds very appealing.

Christina is being chaperoned by her Aunt Patricia who wants Christina's money. Patricia goes to great lengths in an effort to achieve that. She arranges for Christina's kidnapping but Lyon discovers the plot and encounters the men before they can carry out the plan. Eventually, Christina and Lyon are forced to confront Christina's father who confessed to murder.

Although she began the marriage planning for a short-term arrangement, Christina is soon in love with Lyon and enjoys a love that is fully returned. There are compromises along the way as Christina learns to live in the world of the white man and Lyon learns that Christina is like no one that he has ever encountered.