In a futuristic outer space, the commercial towing ship The Nostromo is returning home to Earth with twenty million tons of mineral ore. On board, “Mother,” the aircraft’s computer, awakens the crew of five men and two women from hyper-sleep. After breakfast, Dallas, the captain, learns that company headquarters has ordered the crew to detour and investigate a mysterious audio beacon. Parker, the ship's engineer, gripes that he is not being paid for rescue work, but Ash, the science officer, reminds him that according to their job contracts, they are obligated to look into transmissions of possible intelligent origins and by refusing, they forfeit their monetary share from the mission. Dallas agrees with Ash, and the crew proceeds to disengage the ship from the docking platform and navigate towards the signal. The ship incurs damages from a rough landing on a rocky planetary terrain. While Parker and his technician, Brent, make repairs, Dallas, along with executive officer Kane and navigator Lambert, don spacesuits and track the signal on foot. After coming upon a derelict spacecraft, they climb inside and discover a large alien life form, fossilized in a chair. Meanwhile on board the ship, warrant officer Ripley learns that the signal is a warning rather than an S.O.S. After being lowered into a cavernous chamber, Kane discovers egg-shaped forms covered by mist. He notices movement inside an egg, but while observing, an organism attaches itself to the front of his helmet, leaving him unconscious. Dallas and Lambert carry him back to the ship, but Ripley reminds Dallas that the quarantine law requires twenty-four hours for decontamination and denies them reentry. Overhearing the exchange, Ash ignores Ripley's seniority and opens the hatch. In the infirmary, Ash and Dallas contemplate whether to detach the organism covering Kane’s face. Although Kane is still alive, the organism has paralyzed him. When Dallas cuts off a digit of the tentacle, an acidic liquid drips on the floor and begins to burn through flooring of the decks, temporarily threatening the hull. Unable to assist Kane for now, the crew waits while Parker and Brett repair the ship. Back in the infirmary, Ripley criticizes Ash for letting the organism onboard, which put the entire crew at risk. Ash claims that he disregarded the quarantine law to save Kane’s life. Sometime later, the organism detaches from Kane’s face and disappears. When its lifeless form drops from the ceiling, Ash is eager to conduct tests, while Ripley suggests discarding it. Because Ash is the science officer, Dallas lets him decide. Outside the infirmary, Ripley confronts Dallas to warn him that she does not trust Ash. With the majority of repairs complete, the ship takes off and returns to the docking station. As they continue the journey home, Kane regains consciousness; but during a meal, he begins to choke and convulse. From his chest, an embryonic organism suddenly bursts out and scurries away. After the crew buries Kane by propelling his body into space, they divide into two groups to locate the alien with a tracking device, nets and electrical prods. During the search, the team of Ripley, Parker and Brett, mistake the movement of Jones, the ship's cat, for the alien. When Brett follows the cat into another section, the alien, who has grown large and menacing, jerks him into an air shaft. Ripley and Parker rush in to witness Brett's death. In reporting to Dallas, Parker speculates that the organism is using the air ducts to move around the ship. Therefore, Dallas devises a plan to trap the alien in the main air lock and set it on fire. While the others monitor the alien’s movement and close off vents, Dallas enters the air shaft with a flamethrower. As he approaches a junction in the passage, Lambert yells that the organism is moving toward him. A screech is heard, followed by static on the radio. Parker finds no sign of the captain's dead body. The remaining crewmembers are tense as Ripley decides to continue with the plan to trap the organism in the air shaft. She becomes more frustrated by Ash’s seeming lack of concern or advice. While Parker is refueling the flamethrower, Ripley, as the new commanding officer, consults with "Mother" and learns that the priority is to bring back the alien species; however the crew is expendable. As a distressed Ripley leaves the computer annex, Ash tries to kill her. Lambert and Parker arrive and struggle to fight him off, until Ash begins to spew goo and disintegrate. When his head becomes detached, Parker and the others realize that he is a robot. Because Ash has been protecting the alien all along, Ripley believes that the company wants the species for their weapons division. She repairs Ash's communication function in the hope of extracting knowledge about how to kill the alien. However, as his head returns to life, Ash merely states that they cannot kill a perfect species and admits that he admires the alien for its purity as a survivor. After disconnecting the robot, Ripley decides that they will blow up the ship and escape in the shuttle. While Lambert and Parker collect supplies, Ripley prepares the shuttle for departure. She hears Jones the cat meowing and secures him in a cage. As Lambert and Parker are loading a cart, the alien attacks them. Ripley hears their distress over the radio and rushes to help, but finds both of them dead. After she runs back to program the destruction of the ship, the voice of "Mother" begins the countdown to detonation. Despite the nearby presence of the alien, Ripley and Jones eventually manage to board the escape shuttle without interference from the creature. From the window of the shuttle, Ripley watches the ship disintegrate and announces that she got the “son of a bitch.” She comforts Jones before placing him in the sleep capsule. While setting the controls for the journey home, the hand of the alien unexpectedly lurches out from the instrument panel. Ripley darts into a closet and puts on a spacesuit while keeping an eye on the alien's location. Returning to the control room, she opens the hatch, spears the organism, and it is jettisoned into space. Just as the creature is almost sucked back into the exhaust, she blasts the jet engines and the blaze pushes the alien away from the shuttle. As the only surviving crew member, Ripley dictates a report before entering hyper-sleep, estimating that she will arrive on the frontier in about six weeks.