One of the crew members on the doomed spacecraft Nostromo calls the creature in their midst a "perfect organism. Its structural perfection is matched only by its hostility." Which makes the alien an ideal movie monster in this beautifully designed, symphonically paced, all-time scary science-fiction thriller, scripted by Dan O'Bannon and directed by Ridley Scott. The crabby Nostromans are, in a way, the alien's pediatricians: they see it in five gestations, from its incubation through birth (bursting out of a crew member's chest), childhood and adolescence, into its final mature grandeur. Sigourney Weaver survived to star in three sequels, but the actors who had to die all got their moments. Our favorite: Harry Dean Stanton taking the wrong turn looking for the company cat. "Here, kitty kitty."