George "Sul" Sullivan is a bounty hunter with a magical stone for a heart.
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Unfeeling and efficient, he nevertheless saves the life of Psyche Kyteler, a gunmaker who was assaulted, staked out, and trapped on a deserted ranch. Psyche and Sul take on their mutual enemies together in a desperate stand. Then they part, Sul leaving his stoneheart with Psyche to keep her trauma at bay.
 
Unbeknownst to Sul, Psyche is pregnant with a magical child, the result of her assault and contact with the stoneheart. During the years they spend apart, she raises her beloved daughter and yearns for the man who cannot feel for her as she feels for him.
 
Meanwhile, Sul increasingly acts out the guilt and despair he tried to escape by taking the stoneheart. At last, in desperation, he takes a final job that brings him back into Psyche's life. But far from easing his torment, this job confronts him with an impossible choice between honor and salvation.