In a grim, rain-soaked city, a serial killer called 'John Doe' is at large. He has a taste for literary allusion, and his victims are butchered in grotesque accordance with the dictates of the seven deadly sins. The cops investigating the case are Somerset, a grizzled old veteran on the verge of retirement, and Mills, a fresh-faced youngster with a fretful wife who years for them to leave the city behind. Somerset and Mills get a scent of John Doe's trail, but are unable to pin him down; until the killer walks straight into their hands, and reveals the method in his madness with one last, awful stroke of inspiration.