After their father's death, sisters Vanessa, Ellie, and Georgiana are stunned to learn a well-kept family secret. A yellowed letter tucked away in a family Bible reveals that the woman who gave birth to their late father had not died when he was born, as they had always believed. Their paternal grandmother gave up her son shortly after his birth, leaving this note with instructions to never reveal her name or background to him. The letter is signed, simply, Hattie.

The sisters are each at a turning point -- Vanessa is feeling stifled in a stale marriage and career; Ellie is perpetually single but desperately wants marriage and motherhood; and baby sister Georgiana is a successful model but longs to do something more serious with her life. Seizing upon the idea of finding their long-lost grandmother, the women set out on a trip to Montana, where they hope not only to find their father's birth mother but also to rekindle their bonds of sisterhood and possibly even find their true selves.

But they have no idea of the consequences of their quest. If the sisters find Hattie and live to tell their story, everything Hattie has worked and dreamed for will be destroyed. Will she allow the sisters to uncover the family secret and escape alive?