Twenty years ago, Ellie Enderlin’s sister, Lila, a brilliant mathematician, was murdered in a crime that was never solved. In the aftermath of her sister’s death, Ellie entrusted her most intimate feelings to a man who turned the story into a best-selling true crime book, a book that both devastated her family and identified one of Lila’s professors as the killer. Decades later, Ellie receives a piece of evidence never found with her sister’s body: a notebook of Lila’s filled with mathematical formulas. It will lead her to a hundred-year-old mathematical puzzle, to a lover no one knew Lila had, to the motives and fate of the man who profited from their family’s anguish, and to the deepest secrets even sisters keep from each other.