It’s 1847 in upstate New York, and in the midst of a typhus epidemic, no one can keep their word. 

Florence Hill, a stubborn inn maid averse to change, promised her little sister the same peaceful childhood she remembers. Yet as her family crumbles around her, she finds herself struggling to keep that vow. When a co-worker falls ill and yellow flags appear across town to mark the infected homes, the fragile stability she has clung to is shattered. 

Jesse Jenkins breaks his vow never to attempt a medical procedure again. After a decade of feverish book study, his mentor drags him to the countryside where he will be forced to practice medicine for the first time since the accident that killed his brother. He knows he must return home as quickly as possible or risk making another deadly mistake.

Florence’s mother matches her with William Becker, a well-to-do newspaper editor whose wealth may be the only way to give her sister the childhood she deserves, but the more time Florence spends with Jesse, sharing late-night confessions as the death toll around them rises, the more she risks her engagement and puts her sister’s happiness at stake. When William falls ill and is quarantined in the medical tent, it leaves Florence and Jesse with a little too much time together, and Jesse asks her for one last promise. Now she is faced with the devastating decision of a broken promise to her sister or a broken heart of her own.