The science fiction/horror film Alien (1979), directed by Ridley Scott, has become extremely popular and influential, and has spawned several sequels and imitators. The title of the film refers to highly aggressive extraterrestrial creatures (unidentified in this original film, but referred to and categorized as "Xenomorphs" in the sequel Aliens). But the connecting thread becomes the saga of Ellen Ripley, played by Sigourney Weaver, a human woman who finds herself the principal opponent of the species throughout the series. The film launched the first major American film series with a female action hero.