The film's central theme is the obsessive pursuit of justice, as embodied by the three main characters: Arthur Leigh Allen (Gyllenhaal), a cartoonist who becomes convinced that he has identified the killer; Paul Avery (Downey Jr.), a journalist who teams up with Allen to track down the Zodiac Killer; and Dave Toschi (Ruffalo), a detective with the San Francisco Police Department who leads the official investigation. As the story unfolds, the lines between obsession and sanity become increasingly blurred, leading to a thrilling climax that will keep viewers on the edge of their seats.