Boston, 1846. 18-year-old Mildred Parish, a barber's daughter, practices practical witchraft using locks of hair obtained from her father's customers. She's very selective about who knows her secret and the kinds of spells she casts. Only people she trusts can know and she must never cast a spell to harm another person.
One of her father's clients is Theodore O'Brian, an Irish immigrant whose family is fortunate enough to be wealthy. Mildred is head-over-heels in love with him, but he's destined to be with someone else. One day, a woman named Trinity Hartell comes knocking on Mildred's door. She has a vendetta against an entire family and wants Mildred to cast a death spell on them. The family? The O'Brians, including Theodore. Mildred refuses, but Trinity is set on getting what she wants, one way or another.
Mildred now feels she must protect the O'Brian family and the man she loves, but she must also protect herself. How can she make sure Trinity is stopped without telling the entire city of Boston that she's a witch?