Based on James Ellroy's novel Blood on the Moon, Cop is a grim, brutally violent, darkly humorous modern-day film noir. Lloyd Hopkins (James Woods), is an obsessive, amoral LAPD police detective investigating a murder he believes to have been the work of a serial killer. Hopkins is cynical and obsessed with the way society fills women's heads with fairy-tale promises of romance. "Innocence kills," he sneers. "I see it every day." His investigation leads him to the bookstore of a writer of feminist poetry (Lesley Ann Warren) who has for some time been receiving gifts of poems and flowers from an unknown admirer. Hopkins, looking through her diaries, realizes that the dates of the gifts correspond to the dates of the murders, and he begins a hunt for the killer which leads to a violent and exciting conclusion. Cop is completely absorbing because of Woods' chillingly effective performance. Few actors can make an amoral, clever, sardonic, and vicious character as appealing as Hopkins. As Hopkins, Woods combines complex contradictions with ease, showing the various sides of his character's personality. Cop, while singularly unpleasant is always insightful and fascinating.
~ Linda Rasmussen, All Movie Guide