In 1862, a young newlywed is executed for treason trying to free her husband, a Confederate cavalry officer, from an island prisoner of war camp in Boston Harbor. For years after, soldiers and later tourists swear they have seen a Lady in Black robes wandering the fortress in search of her dead husband, who accidently died by her own hand during the escape attempt. In present times, when a teacher is murdered while taking his history class on a field trip to the old fort, now a national historical monument, witnesses claim to have seen a mysterious person in black near the murder scene. Investigation of the murder falls to the Massachusetts State Police and a veteran detective who is recovering from an opioid addiction and concealing a dark secret which drives him to seek psychological help. The search for the killer leads the detective and his female partner to discover a plot by a Mexican cartel to use the island fortress as a distribution point to flood the American Northeast with counterfeit opioids. Skeptical of ghost stories, the detective, Vince Magliore, nevertheless encounters some inexplicable events while on the island fortress, and in a gun battle with cartel operatives, may have been saved by the intervention of a shadowy apparition. Urged by his psychologist, Magliore travels back to California to confess to his old boss, the chief of his former police department, the secret he has been hiding. The chief’s response to Magliore’s confession is totally unexpected.