The Nostromo glides silently through space as the primeval terror quietly lurks on board. Dallas uses 'Mother' - the ship's computer to evaluate their current procedures to terminate the Alien or other options for possible procedure, but receives unfruitful responses in greenish letters: "Unable To Compute," and "Available Data Insufficient." Finally, he types in: "WHAT ARE MY CHANCES?" The answer returns: "Does Not Compute." He leans forward and puts his hand to his forehead, perplexed at the answers.

Dallas crouches down and advances through the dank air shafts and octagonally-shaped air tunnels with a flame-thrower made by Parker and a light, while Ripley closes the irises of the hatches/air locks behind him as he moves ahead. Lambert tracks his progress on a screen with a bluish background and white grid - a beeping blip identifies Dallas' position. His breathing is heavy on the soundtrack. After Dallas has passed through the third juncture, Lambert warns him to be careful. He experiments with his flame-thrower by sending out fiery bursts, illuminating the dark corridors. But he is gripped by confusion and terror as Lambert loses his signal and interference affects the tracker's signals.

Dallas unexpectedly slides his hand into a strange, slimy, gelatinous substance on the floor. She panics: "Look around. Are you sure that it's not there? I mean, it's got to be around there somewhere...Dallas, are you sure there's no sign of it? I mean, it is there. It's gotta be around there!" Then, the tracker screen shows a second dot moving ominously straight toward him: "Oh God, it's moving right towards you. Move! Get out of there!" The Alien attacks with two hands upraised when Dallas turns and shines his light onto it. The monitor screen ends its transmission with static and a very high-pitched whine, as Ripley screams on her headphone: "Dallas!"

In the next scene, Parker loudly slams Dallas' incinerator gun onto a table in the control room. He reports that there is no evidence of Dallas - "No blood. No Dallas. Nothing!" - only his gun lying at the scene. Ripley, now first in command after Dallas' shocking, unexpected death, takes charge of the frantic, decimated crew. Should they proceed with Dallas' plan, or abandon the ship, taking the small shuttle craft and fleeing into space, hoping to be picked up?:

Ripley: Unless somebody has got a better idea, we'll proceed with Dallas' plan.
Lambert: What? And end up like the others? Oh, no, you're out of your mind.
Ripley: You got a better idea?
Lambert: Yes. I say that we abandon this ship. We get the shuttle and just get the hell out of here. We take our chances and just hope that somebody will pick us up.
Ripley: Lambert, the shuttle won't take four.
Lambert: Well then, why don't we draw straws...
Parker: I'm not drawin' any straws. I'm for killin' that god-damned thing right now!
Ripley: OK. Well, let's talk about killing it. We know it's using the air shafts. (raising her voice) Will you listen to me, Parker?! SHUT UP!
Parker: Let's hear it.
Ripley: It's using the air shafts. That's the only way. We'll move in pairs. We'll go step-by-step and cut off every bulkhead and every vent until we have it cornered, and then we'll blow it the f--k out into space. Is that acceptable to you?...Obviously it means killing it. But we have to stick together.

Ripley is angered by Ash's lack of solutions and inability to neutralize the Alien - Ash has conveniently remained quiet, out-of-focus, and in the background of the frame. He answers her with his back turned:

Ripley: Ash?! Any suggestions from you or Mother?
Ash: No, we're still collating.
Ripley: (laughing in disbelief) You what? You're still collating? I find that hard to believe.
Ash: What would you like me to do?
Ripley: Just what you've been doing, Ash, nothing! I've got access to Mother, now, and I'll get my own answers, thank you.
Ash: All right. (He turns and gives her a rushed, official salute.)

She visits the ship's computer 'Mother' to get her own answers. She types in: "Request Clarification on Science Inability to Neutralize Alien." It responds: "Unable to Clarify." She astutely discovers, with an enhancement, that the computer has been given: "Special Order 937 - Science Officer Eyes Only." With an "Emergency Command Overide," she learns to her horror what Special Order 937 commanded:

Nostromo rerouted to new co-ordinates. Investigate life form. Gather specimen. Priority One. Insure return of organism for analysis. All other considerations secondary. Crew expendable.

Ripley now understands that their true mission all along was to recover this new, apparently indestructible life form. As she drops her head back in disgust, Ash's profile slowly appears behind her, and smugly suggests: "There is an explanation for this, you know."

Ripley is exasperated and enraged, grabs Ash and screams: "I don't want to hear your goddamn explanations!" while pushing him against the computer room's wall. She tosses him aside and rushes to the control room to alert the others over the intercom. Ash scrambles along behind and locks two hatch doors to bar her advance. A drop of white blood trickles down from the side of his forehead - he appears damaged, dysfunctional, and not completely sane. Maddened, Ash lunges at her with super-human strength, tears hair from her head, throws her against a wall, and knocks her unconscious. Next to a wall decorated with girlie pictures, the synthetic android forcibly stuffs a large rolled-up magazine down Ripley's throat to choke her as she struggles back. [The men's magazine is a symbol of male aggression toward women, and in particular, is a reminder of Ash's lifeless and mother-less existence, and subsequent hostility toward females. The potent manner in which the magazine is rolled up and shoved down her throat suggests a domineering, fellatio-style rape.] Parker and Lambert, who were summoned minutes earlier, come to her rescue. To defend himself, Ash squeezes Parker's chest with his clawed hand, and causes tremendous pain.

In another remarkable scene, Parker strikes a blow to Ash's head with a silver fire extinguisher, sending him smashing into the bulkhead and reeling into an uncontrollable spin while spurting white plastic foam and liquid from his head. His out-of-control body suffers severe spasms. Parker strikes him again, separating Ash's head almost completely from the neck - then he can't believe what he sees: "It's a robot! Ash is a goddamn robot!" On the floor lies a mass of spaghetti intestinal tubing and white gore, with Ash's head hanging down the back of the body. To subdue the android even further, Lambert pierces Ash's back with the electronic prod. Ripley and Parker re-assemble and connect enough of the wiring to the severed head so that they may find out how to kill the Alien:

Parker: How come the Company sent us a god-damned robot?
Ripley: All I can think of is they must have wanted the Alien for the Weapons Division. It's been protecting it right along.
Ripley: Parker, will you plug it in?...'cause he may know how to kill it.

After zapping it, Ash's disembodied head gurgles back in a tinny voice and spews whitish drool. He describes his special 937 orders from the ship's Company and his bizarre appreciation for another alien form:

Ash: Bring back life form, priority one. All other priorities rescinded.
Parker: That's a damned Company. What about our lives, you son-of-a-bitch?
Ash: I repeat. All other priorities rescinded.
Ripley: How do we kill it, Ash? There's gotta be a way of killing it. How, how do we do it?
Ash: You can't.
Parker: Bulls--t!
Ash: You still don't understand what you're dealing with, do you? A perfect organism. Its structural perfection is matched only by its hostility.
Lambert: You admire it.
Ash: I admire its purity. A survivor unclouded by conscience, remorse, or delusions of morality.
Parker: Look, I've, I've heard enough of this and I'm asking you to pull the plug. (Ripley reaches forward to silence him)
Ash: Last word. I can't lie to you about your chances, but... (he cruelly smirks at them) you have my sympathies.

Ash has made it diabolically clear that the ship's human crew are expendable and face extraordinary odds in their coming battle against the uncaring and hostile machinations of Mother, the Company, and the Alien itself - all sinister entities "unclouded by conscience, remorse, or delusions of morality." With his last words, Ash is destructively unplugged. Ripley announces their next step: "We're gonna blow up the ship. We'll take our chances in the shuttle. Blow up the ship." Parker blasts the remains of Ash's head and body with the incinerator gun, and the flames melt it down to a plastic skull.

Parker explains that after all the switches are thrown to blow up the ship, the crew has only ten minutes to escape: "If we ain't out of here in ten minutes, we won't need no rocket ride through space." Parker and Lambert search for as much coolant as they can carry for the shuttle's air support system, as Ripley prepares the shuttlecraft Narcissus for its flight. She orders: "In about seven minutes, come back up here. I'll shut the switches off and we'll blow this f--ker off into space." To show that she means business, she ties her hair up on the back of her head. And then she hears a faint, plaintive meow at a distance.

On another darker level of the huge ship, Parker and Lambert load and then noisily push a trolley carrying cylinders of coolant. Elsewhere, Ripley locates the cat's carrying cage and frantically begins searching for Jones (so that the cat won't be left behind) as the suspense builds. When the cat leaps out at her and scratches her, she screams, but she is able to capture the spooked cat in her hands and place it in the cage.

Shadows appear over Lambert's figure as she stockpiles the cylinders, and she is paralyzed by the sight of the alien creature. From behind, Parker defends her, screaming: "Get out of the way," but he is unable to use his flame-thrower. Over the intercom system, Ripley hears their horrified shrieks as the creature attacks both of them and places them in deadly peril. Parker suffers a painful, bloody death when the creature turns first toward him, holds him in the air, and bares its ugly mouth and teeth. Ripley runs through the ship's corridors to help them, as the adult form of the Alien now turns toward Lambert and moves one ominous, sinuous tentacle around her ankle and back and disappears up between her legs [more sexual symbolism]. After more disturbing shrieks (especially from Lambert) are heard over the radio, there is dead silence - Ripley arrives too late. She discovers their grisly remains - Lambert's bloody leg and foot are hanging in the air, and Parker's body is slumped-over. Panicked, but with tremendous reserve, Ripley is now more determined than ever to eliminate and destroy the Alien on her own - she races back down through the corridor while gasping for air.

[Following this scene is one of the most notoriously deleted scenes in the film that was cut for the theatrical release - it was restored in a later director's cut. The scene features Ripley entering the landing gear area of the Nostromo (where Brett was killed), and discovers a cocooned Dallas and Brett transforming into an Alien egg. Dallas, still alive and conscious, pleads: "Kill me!" Ripley then mercifully uses her flamethrower to kill Dallas and the "Brett-egg". This cut is significant in that (1) the fate of Dallas is openly revealed, and (2) later sequels, beginning with Aliens (1986), would introduce the concept of the giant Alien Queen, who lays eggs with an ovipositor, rather than Alien victims transforming into eggs.]

Methodically, she starts each phase of the detonation procedure, at the start of the film's exciting conclusion. She opens a control panel which sets off a steady beeping sound, and then pulls two large red switches with sweaty hands. A heavy metal panel opens on the floor - under its cover is a warning which reads: "DANGER - Emergency Destruction System - On Activation Ship Will Detonate in T Minus 10 Minutes. Failsafe Warning - Cut-Off System Will Not Operate After T Minus 5 Minutes." She activates the system (to turn off the coolant system, causing overheating) by punching buttons and setting four cylinders in the control panel. 'Mother's' female voice warns:

Danger. The emergency destruct system is now activated. The ship will detonate in T minus ten minutes. The option to override automatic detonation expires in T minus five minutes.

Sirens honk loudly, jets of hot steam pierce the ship, and strobe lights flash. Ripley moves quickly toward the shuttlecraft and retrieves the cat carrying cage on her way. Two minutes later, she is jolted to find the Alien blocking the only passageway to the shuttlecraft. Seemingly trapped, she panics, sets the cage down, and retreats back to turn off the detonation in a race against the clock. The Alien is intrigued as it surveys the marmalade-colored cat in its cage.

When the loud countdown reaches zero seconds left to override the automatic detonation (after counting down from 30 seconds), Ripley realizes the ship will destruct in five minutes and the override option has expired. Missing the deadline to shut off the ship's self-destruct, she angrily screams out (with a maternal analogy in her command, as if she is a chick being forcibly pushed out of her nest) - to no avail:

Mother! Turn the cooling unit back on! Mother!...You Bitch!

With a flamethrower in her hand that spouts a bit of fire, she rushes back to the shuttlecraft, now hearing that she has only one minute left to abandon ship. With the Alien nowhere in sight where she left the cage, she cautiously enters the shuttlecraft just as a series of minor explosions begin to shake the Nostromo at T minus one minute.

Frantically, she proceeds through the shuttlecraft's purge/launch procedures, as she listens to 'Mother's' final coundown. Ripley hits switches on the controls and straps herself in. At exactly 25 seconds until detonation, the Narcissus is launched and pulls away from the mother ship - the underside of the mother ship becomes a blur as it passes above the shuttle during the take off. Behind her, there is a blinding flare of horizontal light as Ripley watches the Nostromo blown into oblivion - numerous explosions and their impact rock the shuttle. After passing beyond the intense blasts into the quiet and blackness of space, a satisfied, relaxed Ripley mumbles softly to herself: "I got you! You son-of-a-bitch." The resourceful heroine, the only remaining crew member, believes she has just eliminated the Alien forever.

Ripley must now prepare for hyper-sleep and her long, uncertain journey home. She unstraps herself, reassures Jones in her arms, opens the clear lid on one of the padded bunk-beds, and places the cat inside. Ripley removes her gray outer clothing down to her skimpy, pale green t-shirt and brief bikini panties. Disrobing even further, she strips down to a half white t-shirt on top. [Some criticized Ripley's strip-tease as exploitative, although it reinforces the idea that she is the only survivor - a full-bodied female.]

As she flips switches, she is suddenly stunned to have the creature's hand/claw pop out in front of her in the cramped control cabin. Ahead of her, the intelligent alien had found a hiding place in the shuttle to escape the explosion in the main ship. She scurries to the equipment/wardrobe locker, simultaneously trembling and deciding on her next move. She cautiously and methodically places one long leg, and then the other, into the shuttlecraft's spacesuit hanging next to her. The Alien moves in the maze of machinery and bares its deadly teeth to her like a voyeur. She dons a helmet and grabs a harpoon gun to load it. She nervously sings "Lucky Star" under her breath to bolster her confidence over her nightmarish dilemma: "You are my lucky star...Lucky, lucky, lucky, lucky, lucky."

She slowly straps herself in and pushes buttons to release hot gases that she hopes will force the Alien to leave its hiding place and move into position so that it can be blown free from the craft. After two unsuccessful button pushes, the third one finds its target and forces the squealing creature out into the open. She turns her head sideways in her helmet to witness the creature moving menacingly closer and closer to her and revealing its double and quadruple rows of metallic teeth. Ripley slams her hand down hard on the activator button, opening the hatch and depressurizing the cabin. The air whooshes from the craft as the Alien is jettisoned backwards out the hatch's door - but it is caught spread-eagled in the opening. She aims and fires a target gun at its belly, forcing it deeper into space. The hatch door shuts but catches the tip of the gun, leaving the Alien suspended and attached to the craft like an umbilical cord.

The Alien's survival instinct takes over - it bounces on the side of the shuttle and attempts to board the craft through the engine exhaust. Ripley outwits the Alien and blasts it with the white heat of the exhaust into space. She watches the last remains of the scorched Alien disappear. The space ship is finally purged of the poisonous, destructive intruder. She makes her final report while stroking Jones in her lap - note how she pauses, disgustedly, when including traitorous android Ash in the list of casualties:

Final report of the Commercial Starship Nostromo. Third Officer reporting. The other members of the crew, Kane, Lambert, Parker, Brett, Ash and Captain Dallas are dead, the cargo and ship destroyed. I should reach the frontier in about six weeks. With a little luck, the network will pick me up. This is Ripley, last survivor of the Nostromo, signing off.

She, and Jones, enter hypersleep for the long journey home - the last image is of her sleeping figure underneath the lid of the padded bunk, with soothing lullaby music playing to provide a catharsis to the previous action-packed sequences.