Darker than Black is a science fiction anime series set in a world where mysterious spatial anomalies called 'Hell's Gate' and 'Heaven's Gate' have appeared, giving rise to individuals known as Contractors who possess supernatural abilities but lose their humanity. The story follows Hei, a Contractor known as the 'Black Reaper', as he navigates the shadowy world of espionage and secret organizations. The series is renowned for its dark atmosphere and complex plot.