The Alien is first seen as a Facehugger on the Derelict ship, where it facehugs Kane and impregnates him with a Chestburster. After erupting from Kane's chest, the Alien runs away and then grows. It later encounters Brett, where it headbites him and almost instantly killing him, dragging him away. After a breif argument, Dallas decides to hunt the Alien in the USCSS Nostromo's vents. While Dallas hunts the creature in the vents, Lambert, Ripley, Ash and Parker track it. Lambert loses the Alien, and both she and Dallas begin to panic. In the confusion, Dallas is ambushed by the Alien leaving no trace of what it did to him.
The crew decides to resume with Dallas' plan. With both Kane and Dallas becoming victims of the Alien, Ripley is now in charge of the crew. After accessing Mother and discovering a secret message directed to Ash stating "Bring back alien life form. Crew expendable." Ripley is then ambushed by Ash, but then saved by the crew as they discover that Ash is a synthetic. With only 3 people remaining, they settle with Lambert's plan; Cause the Self-Destruct sequence and escape in the escape shuttle Narcissus. While Ripley gatheres food, codes, and Jones the Cat, Lambert and Parker are sent to get coolant. While getting the coolant The Alien attacks Lambert and Parker. It begins to attack a fear-paralyzed Lambert, but is interrupted by Parker's attempt to kill it. Knocking him away with its' tail, The Alien pin him to the ground and headbites Parker while he screams, telling Lambert to run. Lambert, however, paralyzed by the presence of the creature, remains where she was, and is raped by the Alien's tail. (This death is not shown, but it is heard over Ripley's radio) With nobody to save or wait for, Ripley immediately starts the Self-Destruct sequence. On her way to the shuttle, Ripley encounters the Alien. She drops Jones and runs away to deactivate the Self-Destruct sequence, unsure if she would have enough time for the Alien to move away.
After failing to turn off the sequence in time (mostly due to MU-TH-UR's (Mother's) refusal to deactivate the sequence), Ripley runs and checks for the Alien. Fortunately, it is gone, and Ripley makes it to the Narcissus. After escaping an explosion of well over several hundred megatons, Ripley relaxes. She is then horrified after the Alien's hand lashes out and then draws back in, The Alien itself asleep aboard the ship that it had escaped onto. Trapped with the very monster that had murdered her friends, Ripley tries to kill the Alien via burning it with steam. This fails, and the Alien rears up to her to headbite her. Ripley screams and open the airlock. While Ripley herself is locked to the chair, the Alien is loose, and gets sucked into the vacuum. However, it is able to withstand the vacuum and begins coming back into the ship. Fortunately, it is hit by something heavy enough to knock it off balance and make it fall out. The Alien drifts out into space an latches onto the engine with its' tail. Ripley turn the engines' afterburners on, incinerating the Alien and blowing it away into deep space.
The Alien's purpose was to attack the audience in a way nobody would think of, and this attack was directed HEAVILY towards the Male gender, although the female gender was also given a similar attack. This made Ridley Scott and Giger decide on making the Alien a symbol for two fears. The Fear of the Unknown, and the Fear of Sexual actions. The Facehugger impregnating Kane was symbolizing homosexual oral rape, and the Chestburster was symbolic for a man giving birth. Lambert's death was a fear for women, the fear of being forcefully raped. In a deleted scene of the Alien's attack on Lambert, the Alien pointed its tail at Lambert in an erectile manner, symbolizing a penis (This gives reason to believe that the Aliens are a genderless species as they bear several male traits as they do female.) This scene became known as the infamous "Crabwalk scene" which was deleted because it showed to much of the Alien. In another deleted scene, it is shown what actually became of Dallas and Brett after the Alien captured them. In this scene Ripley walks down a ladder and finds Dallas and Brett, in a mini-hive set up by the Alien. The Alien has been turning them into Facehugger eggs, which is directed towards the "Man giving birth" attack. This was deleted probably because the Eggs' origin may have been supposed to stay a mystery.
Ridley Scott also decided to add to the creature's mysterious quality by refusing to let the audience see to much of it. In the Director's Cut, the Alien can be seen nearly completely in Brett's death, and is not seen that way again until the end of the movie, where the entire body is revealed before its' own death and even then in a vary hazy fashion, due to it's black body blending in with space and the afterburner fogging up the screen, we still are only allowed to see the general shape of the Alien and from the back at that.