Twenty years after her sister, Lila, is murdered, Ellie Enderlin has a chance encounter with the man accused of the crime, who gives Ellie her sister's notebook, filled with mathematical equations and clues that will lead Ellie on a journey into the past that offers startling revelations about her sister's life.

"A widowed FBI agent grows suspicious of her son's new school in this thriller by the New York Times bestselling author of The Marriage Pact. Lina is on leave from her job in New York at the FBI in order to clean out her father's home in Silicon Valley. As though letting go of her father isn't hard enough, Lina has also recently lost her husband in a freak traffic accident. Still reeling, she and her teenage son Rory must make their way through this...