In the 1880's Caroline Trewartha became a mining engineer by acting as a boy in Pennsylvania's most prestigious mining school. By 1888, at twenty-four, she is too obviously a lady to play the man, yet her company assigns her to improve operations at a mine they have loaned startup money, the only anthracite mine outside of the east. Anthracite is rare and very valuable.
Arriving in By-Gum Mines, Colorado, she is not surprised that the darkly angry mine owner, Gryffith Williams, doesn't believe her credentials. However, she soon also discovers a deeper reason for his distrust than her sex. Someone has flooded his mine and killed his good friend. He believes her company sent her to By-Gum to destroy him and take over his company. When two more friends are murdered, she convinces Gryf to accept her help, whether or not he trusts her.
As she works to discover who is responsible for deaths and attempts to ruin the mines, Caroline clashes with Gryf and occasionally with those in his crew who believe women should never be in a mine. As Gryf learns to trust her, and the women of By-Gum come to accept and love her, Caroline feels she has finally found a possible home. Then, she discovers too much, and becomes the next target.