Whether it's a cheating ex, a double-crossing co-worker, or the best friend who stabbed you in the back, it's human instinct to want to get even. Though most people don't act on this impulse, there are those who are consumed with getting even. For them, what starts as a nagging bitterness evolves into obsession and results in murder. This true crime documentary series profiles those stories of revenge, told through gripping interviews with the family members, community members, and law enforcement who experienced the events firsthand.

A mother, herself retired from the police, embarks on an underground exploration of Paris to find her daughter, a missing investigator on a mission.