Dr. Olivia Simon is on duty in the emergency room of North Carolina's Outer Banks Hospital when a gunshot victim is brought in. Midway through the desperate effort to save the young woman's life, Olivia realizes who she is-Annie O'Neill, the woman Olivia's husband, Paul, is in love with.

When Annie dies on the operating table, she leaves behind three other victims: Alec O'Neill, who thought he had the perfect marriage; Paul, whose fixation on Annie is unshakable; and Olivia, who is desperate to understand the woman who destroyed her marriage. Now they are left with unanswered questions about who Annie really was-and about the secrets she kept hidden so well.