In the infirmary, Kane is stretched out on a table. [In the original director's cut, Lambert briefly and tearfully attacks Ripley for her actions at the airlock.] In a quick-cut to this new scene, Ash and Dallas, in close-up, use a laser blade to cut through the clear front of Kane's mangled helmet and split it into two halves. They view the quivering, pulsating, fleshy creature which has long digitals or claws, a reptilian tail (tightly wrapped twice around Kane's neck), and a body which covers Kane's face like a gas mask. Dallas exclaims: "My God! What the hell is that? We gotta get that off him." The other crew members are watching from behind the protection of a gallery window. But Dallas stops Ash from cutting off one of its fingers: "It's not comin' off without tearing his face off with it." Parker asks: "How come they don't freeze him?"

Dallas and Ash view an M.R.I. internal scan of Kane's entire body, after which Dallas rhetorically asks about the intrusive creature: "What's it got down his throat?" Somehow, the pulpy mass of organic life feeds Kane with oxygen. They notice that the thing "paralyzes him, puts him in a coma, then keeps him alive. What the hell is that?" Dallas emphatically decides to cut it off of Kane, although Ash cautions: "Let's not be too hasty. We don't know anything about it. Now we assume it's feeding him oxygen. If we remove it, it could kill him." Dallas assumes full responsibility if it kills him: "I'm willing to take that chance - let's cut it off him now."

As Ash makes an incision just by the knuckle of one of the digitals, the creature's blood - an acidic yellow bile liquid - squirts onto the infirmary floor, hisses and immediately eats through the surface and drips quickly down to the next deck. Dallas is startled and runs from the infirmary to alert the crew: "That crap's gonna eat through the hull...That thing's gonna eat through the goddamn hull." Fortunately, it stops smoking and smoldering after passing through a few levels in the shuttle's structure. Brett volunteers his pen for Dallas to stick into the acidic substance: "It must be using it for blood." Parker is amazed: "It's got a wonderful defense mechanism. You don't dare kill it!"

For the time being, however, peace and order have returned to the shuttle. Brett and Parker unsuccessfully attempt to repair it, grumbling: "We shouldn't have landed in this place in the first place." Parker is anxious to get home: "Well, as soon as we patch this thing up and get out of here, the sooner we can go home. This place gives me the creeps." Kane rests comfortably bathed by the white light of the infirmary.

After viewing a video copy of Kane's X-rays, Ash defensively explains his analysis of the alien cell's structure to Ripley in a testy dialogue in which she discredits his scientific approach and his judgment:

Ripley: That's amazing. What is it?
Ash: Oh yes it is. I don't know yet. Did you want something?
Ripley: Yes, I, uh. Have a little talk. How's, uh, how's Kane? (She opens and closes a pair of scissors.)
Ash: Just holding, no changes.
Ripley: And, uh, our guest?
Ash: ...Well, as I said, I'm still collating it, actually, but uh, I have confirmed that he's got an outer layer of protein polysaccharides. He has a funny habit of shedding his cells and replacing them with polarized silicon which gives him a prolonged resistance to adverse environmental conditions. Is that enough?
Ripley: That's funny. What does it mean?
Ash: ...Well, it's an interesting combination of elements making him a tough little son-of-a-bitch.
Ripley: And you let him in!
Ash: I was obeying a direct order. Remember?
Ripley: Ash! When Dallas and Kane are off the ship, I'm Senior Officer.
Ash: Oh yes, I forgot.
Ripley: You also forgot the Science Division's basic quarantine law.
Ash: No, that I didn't forget.
Ripley: Oh I see, you just broke it!
Ash: Look! What would you have done with Kane, hmm? You know his only chance of survival was to get him in here.
Ripley: Unfortunately, by breaking quarantine, you risk everybody's life.
Ash: Maybe I should have left him outside. Maybe I've jeopardized the rest of us, but it was a risk I was willing to take.
Ripley: That's a pretty big risk for a Science Officer. It's, uh, not exactly out of the manual, is it?
Ash: I do take my responsibilities as seriously as you, you know. You do your job and let me do mine, yes? (She turns away and leaves. Ash drinks from a beaker containing a milky-white substance.)

Seeking peace in the quiet of the shuttle craft's control room and listening to Mozart's Eine Kleine Nachtmusik, Dallas is interrupted by Ash's voice on the intercom, summoning him to the infirmary: "I think you should have a look at Kane...Something's happened...interesting." Dallas calls for Ripley to meet him there. The next camera shot is a large side view of Kane's sweat-covered face - the creature has left its perch. The three crew members (Dallas, Ash, and Ripley) hesitantly poke around in corners and upper crevices with probes trying to locate it. In a startling moment, Dallas knocks over a heavy object. As Ripley looks down at Kane on the table, the tail and then the apparently dead carcass of the alien falls on her shoulder and to the floor from overhead.

Ash examines and pokes at its fleshy soft underside - it flinches in a reflex movement: "That's a reflex action. We've got to have a look at this." On a light table, Ash probes under the oyster-like folds and layers of the creature's underbelly: "It looks dead enough." Ripley wants it destroyed: "Let's get rid of it," but Ash shows a morbid fascination for it and is permitted to make the decision to preserve the specimen:

Ash: This is the first time that we've encountered a species like this. This has to go back. All sorts of tests have to be made.
Ripley: (protesting) Ash, are you kidding? This thing bled acid. Who knows what it's gonna do when it's dead?
Ash: I think it's safe to assume it isn't a zombie. (He turns to Dallas for the final decision.) Dallas, it has to go back.
Dallas: Well, I'd soon as not burn it at the stake, but you're the science officer. It's your decision, Ash.

Ripley is upset at Dallas for allowing Ash to have the final say - he hurriedly walks away and won't turn toward her, face her, and answer her confrontational question: "Just tell me how you can leave that kind of decision to him?" She closes a hatchway door on a corridor to halt his getaway (foreshadowing what Ash would later do to her in the film) - he excuses his decision with an exasperated, condescending tone: "Look, I just run the ship. Anything that has to do with the Science Division - Ash has the final word...It happens, my dear, because that's what the Company wants to happen...Standard procedure is to do what the hell they tell you to do." She learns that this is the first time Dallas has shipped out with Ash. Dallas's usual Science Officer that he worked with five previous times was replaced by Ash two days before the Nostromo journey. Ripley is wary: "I don't trust him." He replies: "I don't trust anybody." Projecting his anger onto her, he reprimands her for not informing him earlier that the repairs on the shuttle are nearly completed: "Why the hell didn't you tell me so?...Look, I just want to get the hell out of here, all right?"

Finally, the shuttle lifts off the planet's surface with some momentary difficulties, overheating and shaking. Parker beams happily: "Walk in the park," as they successfully return to the cargo ship Nostromo on its homeward-bound course. The crew is edgy about Kane, with Parker suggesting: "What I think we should do is just freeze him. I mean, he's got a disease. Why don't we stop it where it is? He can always get to a doctor when we get back home." Dallas adds: "Kane's gonna have to go into quarantine, and that's it." Ripley chimes in: "Yes, and so will we." Lambert appears, announcing that they still have a grueling ten month trip back to Earth. Again from the infirmary, Ash speaks to Dallas on the intercom: "Dallas, I think you should come and see Kane." Apparently recovered, Kane seems to have recuperated after the creature has fallen off his face, although he appears to have a stiff neck and has a voracious thirst - he tells the crew his last memory:

Dallas: Do you remember anything about the planet?
Ripley: What's the last thing you do remember?
Kane: I do remember some horrible dream about smothering, and... anyway, where are we?
Dallas: Right here.
Ripley: We're on our way home.
Brett: Yeah, back to the old freezerinos. Ha, ha, ha.
Kane: Oh no, could I have something to eat before we go back to...
Parker: I need something myself.
Dallas: One more meal before bedtime - I'm buyin'.
Parker: Hey, that's a break.

Before they all face a long "bedtime" in the padded bunks, Kane demands to get something to eat at the meal table.

Alien is best-known for the next genuinely shocking and memorable scene. Kane sits eating at the mess table with his crew mates, contemplating his return home: "The first that I'm gonna do when I get back is to get some decent food." Lambert remarks prophetically: "You pound down the stuff like there's no tomorrow." Kane experiences a seizure - coughing and choking on green, spaghetti-like strands of food. His crew mate Parker asks:

What's the matter, man? The food ain't that bad, maybe! What's wrong?

As he rises and struggles, Kane is turned around and laid on the table. Held down by the crew, they force a spoon into his mouth to prevent him from choking on his tongue. And then, in a terrifying moment, blood graphically explodes out of the front of his white T-shirt. As he moans, jerks violently, quivers, and dies, the Alien bursts from the bloody spot on his chest - the hissing, razor sharp-toothed monster/lizard is literally "born" from the guts of the first infected crewman. (Unbeknownst to everyone, the parasitic creature on Kane's face had inseminated him and caused the incubation of a new life form within his 'maternal' body.) Blood splashes everywhere, spraying mostly toward Lambert. Screeching and covered with blood, the fearsome, yellow, snake-like creature spins around in the blood and eagerly looks at everyone. Ash shouts: "Don't touch it," when Parker threatens to kill it with a table knife. The young beast opens its steel, teeth-rimmed jaws and cries out - its birth howl - and then scurries off the table to hide somewhere in the Nostromo. [The chest-burster's scream combines the sounds of a snake, a pig's squeal, and a baby's cry.] Ash follows its path with a look of wonderment and disbelief. The entire crew now faces a homicidal Alien creature that has infested the ship and will stalk them for nourishment.

[The famous chestburster scene would be spoofed in the science-fiction Mel Brooks comedy Spaceballs (1987), in which Hurt, playing Kane, would suffer the same fate in an intergalactic diner, moaning, "Oh, no... not again!" This time, the chestburster would don a top hat and cane, and sing Hello, My Baby while dancing like Michigan J. Frog from the Warner Brothers animated short One Froggy Evening (1955).]

The camera pans around through empty, cavernous corridors of the ship. A few hours later, Kane's body, in white mummy wrappings, is jettisoned from the spacecraft from the inner hatch. The crew witnesses the voiding of Kane's body on a closed-circuit television screen, after Dallas asks perfunctorily: "Does anybody want to say anything?" When he flips the switch, the white object is propelled from the ship and flips end over end into the dark void of space.

A serious search for the Alien begins. In preparation, Brett has fashioned a electrified cattle prod - "it shouldn't damage the little bastard, unless its skin is thinner than ours, but it will give him a little incentive." Ash has designed a short-range tracking device that gives off a signal when sensing "microchanges in air density." [This references the geiger counter in The Thing (From Another World) (1951) - both served the same purpose of tracking the creature and creating additional suspense. The sequel, Aliens (1986), would exploit this plot device to an even greater effect.] And Parker has found a heavy rope netting. Dallas splits the crew into two teams of three members each: Ash, Lambert and Dallas, and Ripley, Parker, and Brett, with the following directions:

Now anyone see this thing or catch it in the net that Parker is holding in his lap - (He interrupts himself with an aside to Parker) Parker - I don't want any heroics out of you, all right? - Catch it and put it in the airlock. Channels are open on all decks. I want you to communicate and keep in touch at all times, all right? Let's go.

Deep in the hold of the ship that is darkened because of a failed lighting system, Ripley, Parker, and Brett creep along cautiously. After some mechanical tweaks to the circuits, Parker restores the lights. Ripley rudely criticizes the mechanism of Ash's tracker device: "Microchanges in the density, my ass!" She also says: "You guys stick together." When the device emits an intermittent sound registering an unknown object beyond the doorway of a locker, they nervously slide the door open with the rope net readied to capture it. In a tense moment, Jones - the ship's pet cat, appears in close-up, with its wide-open mouth hissing at them. The stowaway cat runs by them and evades their awkward efforts. Parker suggests bagging it so that it won't be picked up again on their tracker.

In an unwise strategic decision, Brett goes alone to catch it, calling after it from a distance and following the cat into 'C' level's undercarriage room: "Here Kitty! Here, kitty kitty! Kitty crap. Jones! Jonesy. Here Jonesy. Meow! Meow! Jonesy." He hears the cat 'meow' occasionally, but the independent-minded creature scampers away. Brett discovers the shed skin of the creature on the floor - a sign that it is evolving further and growing rapidly. He pursues the animal into another cavernous area with a cooling tower, where heavy chains dangle and swing from the ceiling, and the sounds of dripping water echo. Brett moves under the cascading rain-water and bathes his face in the cooling moisture. He finally locates the cat hiding in a doorway and coaxes it toward him, but it hisses and recoils at him.

Unbeknownst to Brett, something behind him has caused the cat to react - the movement of a long, coiled tail, and a side-view of the grown monster's head with dripping saliva. He tries to calm the cat: "Hey, I'm not gonna hurt ya." Slowly, he turns around and is overwhelmed as he faces the sight of the alien stalking him, now with metallic teeth glimpsed briefly in rows in its massive jaws covered with glue-like drool. In a shocking moment, Brett is helplessly and brutally murdered by the hungry, malevolent killing machine - Jones calmly watches in a close-up as his body is pierced, bloodied, and taken aloft. [Jones shows no remorse nor concern for Brett's demise, seemingly taking an almost intellectual curiosity in the gory act.]

Sometime later, Parker describes his reactions to the beast - in a full close-up: "Whatever it was, it was, it was big." Dallas conjectures that "it took him into the air shaft," and Ripley adds: "disappeared into one of the cooling ducts" to move around. It is unlikely that Brett survived the attack. Dallas surveys blueprint drawings of the shafts, and under his command, the crew continues to discuss strategies to locate, trap and eliminate the Alien in the air systems:

This air shaft may work to our advantage. Here it leads up to and comes out in the main air lock. All right. There's only one big opening along the way, but we can cover that up and then we drive it in the air lock and zap it into outer space.

Parker isn't so sure the proposed plan will work: "This son-of-a-bitch is huge! I mean it's like a man, it's big." Prompted by Ripley's goading for scientific solutions, Science Officer Ash coldly analyzes the creature:

Ripley: Come on, Ash, I mean, the Science Department should be able to help us. What can we do to drive it?
Ash: Yes, well, it has adapted remarkably well to our atmosphere considering its nutritional requirements. The only thing we don't know about is temperature.
Ripley: OK, what about temperature? What happens if we change it?
Ash: Let's try it. Most animals retreat from fire, yes?
Dallas: Fire, yeah.

Parker is assigned to rig up three or four incinerating units since the alien may retreat from fire and be driven back through the cramped tunnels of the air shaft. Lambert gulps and asks tentatively: "Who gets to go into the vent?" Assigned different duties and positions, Ripley and Ash will take their places in the air lock, and Parker and Lambert will cover up the maintenance opening. Dallas will enter the ventilation system to drive the monster into the air lock.