New York City, 1922.

Millie Carpenter is in the business of mercy. Two years as a field nurse during the Great War taught her much about suffering. Still, when she returns to Manhattan, she employs her mercy in the impoverished tenements of the Lower East Side, where she serves as a Visiting Nurse to Italian immigrants.

Millie's passion for helping people is powerful. But when Maria, a pregnant patient, is suddenly murdered at the hands of her abusive husband, Millie knows what she must do to stop him.

Maria's daughter, Angeline, has been stashed away in an orphanage by her mother's cunning brother, Marco. Millie feels responsible for Angeline and begs to know where she is. Marco agrees to tell her where his niece is under the condition that Millie performs a sinister favor for him. Except one favor turns into two. Then things come to a screeching halt when Marco insists that Millie must kill someone she knows to find Angeline's location.

Millie's once humdrum life transforms into days filled with poison, murder, speakeasies, and dangerous gangsters she begins to call friends. Now, more than ever, she must challenge her idea of right and wrong while trying to save those around her. With an expansive knowledge of pharmaceuticals, Millie finagles the delicate line between tonic and toxin.