The commercial towing spaceship Nostromo, owned and operated by the Earth mega-corporation Weyland-Yutani, is on a return trip to Earth hauling a refinery and twenty million tons of mineral ore, with a seven-member crew in stasis. Detecting a transmission of unknown origin from a nearby planetoid, LV-426, the ship's computer awakens the crew. They set out to investigate the transmission's source. The Nostromo lands on the planetoid.
Captain Dallas, Executive Officer Kane, and Navigator Lambert set out to investigate the signal, leaving Warrant Officer Ellen Ripley, Science Officer Ash, and Engineers Brett and Parker behind on the ship. Dallas' team discover that the signal is coming from a derelict alien spacecraft. Inside they find the remains of a large alien creature whose ribs appear to have been broken outward from the inside.
Meanwhile, Ripley determines that the transmission is actually some type of warning. Kane discovers a vast chamber containing thousands of eggs, one of which releases a creature that attaches to his face. Rendered unconscious, Kane is carried back to the Nostromo. Ash violates quarantine protocol by opening the airlock, allowing the team aboard. After an unsuccessful attempt to surgically remove the creature from Kane's face, the crew discover that the creature's blood contains a corrosive acid.
The creature eventually detaches from Kane's and is later found dead. The Nostromo leaves the planet and resumes its trip. Kane awakens from his ordeal with some memory loss but no other apparent ill effects. During the crew's final dinner before re-entering stasis, he chokes and convulses in pain until a small alien creature bursts from his chest, killing him. The creature escapes into the ship and the crew attempts to locate and capture it by fashioning motion trackers, electric prods, and flamethrowers.
However, the Alien has grown at an unprecedented rate and is now eight feet tall with incredible strength and agility. It attacks Brett and disappears with his body into an air shaft. Dallas enters the airshaft network intending to force the Alien into an airlock, but it ambushes him. When the others find no trace of Dallas, Lambert, implores the remaining crew members to escape in the ship's shuttle.
Ripley explains that the shuttle will not support four people, and recommends that they continue with Dallas's plan of cornering and flushing out the Alien. Accessing the ship's computer, Ripley discovers that Ash has been ordered to return the Alien to their corporate employers even at the expense of the crew's lives. Ash then appears and attacks Ripley, but Parker and Lambert intervene, decapitating and impaling Ash.
Ash is an android programmed to act on pure orders from the company, the survivors destroy it. They agree to arm the Nostromo's self-destruct mechanism and escape in the shuttle, but Parker and Lambert are ambushed and killed by the Alien while gathering necessary coolant supplies. Now alone, Ripley initiates the self-destruct sequence and heads for the shuttle, but finds the Alien blocking her way.
She retreats and unsuccessfully attempts to abort the self-destruct, then returns to find the Alien gone and narrowly escapes in the shuttle as the Nostromo explodes. As she prepares to enter stasis, Ripley discovers that the Alien has hidden aboard the shuttle. She puts on a spacesuit and opens the shuttle's main door, causing explosive decompression which forces the Alien to the shuttle's open doorway.
She propels it out into space by shooting it with a grappling hook, but the gun catches in the closing door, tethering the Alien to the shuttle. As it attempts to crawl into one of the engines, Ripley activates them and blasts the alien into space. She then puts herself and Jones the cat into stasis and resumes the voyage back home.
Ellen Ripley, the only survivor of the space freighter Nostromo, is rescued and revived after drifting for 57 years in stasis. The Weyland-Yutani Corporation meet Ripley's testimony regarding the Alien with extreme skepticism. Ripley loses her space-flight license and learns that LV-426 is now home to a terraforming colony. Some time later, Ripley is visited by Weyland-Yutani representative Carter Burke and Lieutenant Gorman of the Colonial Marines.
They inform inform her that contact has been lost with the colony on LV-426. The company decides to dispatch Burke and a unit of space marines to investigate, and offers to restore Ripley's flight status if she will accompany them as an advisor. Ripley initially refuses, but after recurring nightmares and a promise from Burke that the team will destroy any Aliens found, she accepts. Aboard the warship Sulaco she is introduced to the Colonial Marines.
Sergeant Apone, Corporal Hicks, Vasquez, Hudson, Dietrich, Ferro, Spunkmeyer, Drake, Frost, Crowe, Wierzbowski, and the android Bishop. The expedition descends to the surface of LV-426 via dropship, where they find the colony seemingly abandoned. Two living Face Huggers are found in containment tanks. The only colonist found is a traumatized young girl nicknamed Newt. The space marines determine that the colonists are clustered in the fusion powered atmosphere processing station.
They find a large Alien nest filled with the cocooned colonists. The Aliens then attack, capturing Apone and Dietrich and killing other marines. Ripley is able to rescue the others. With Gorman knocked unconscious during the rescue, Hicks assumes command and orders the dropship to recover the survivors. He intends to return to the Sulaco and destroy the colony from orbit. A stowaway Alien kills the dropship pilots in flight, causing the vessel to crash into the processing station.
The surviving humans barricade themselves inside the colony complex. Ripley discovers that it was Burke who ordered the colonists to investigate the derelict spaceship and he hopes to return Alien specimens to the company laboratories driven by profit incentives. She threatens to expose him, but Bishop soon informs the group that the damaged processing station will soon detonate with the force of a forty megaton thermonuclear weapon.
He volunteers to crawl through the piping conduits to reach and use the colony's transmitter to pilot the Sulaco's remaining dropship to the surface by remote control so that the group can escape. Ripley and Newt fall asleep in the medical laboratory, awakening to find themselves locked in the room with two facehuggers, which have been released from their tanks. Ripley then activates a fire detector to alert the marines, who rescue them and kill the creatures.
Ripley accuses Burke of attempting to smuggle implanted Alien embryos past Earth's quarantine inside her and Newt. He's planning to kill the rest of the space marines in hypersleep during the return trip so that no one could contradict his version of events. The electricity is suddenly cut off and numerous Aliens attack through the ceiling. Hudson, Burke, Gorman, and Vasquez are killed while Newt is captured by the Aliens.
Ripley and an injured Hicks reach Bishop and the second dropship, but Ripley refuses to leave Newt behind. She rescues Newt from the hive, where the two encounter the Alien queen and her egg chamber. Ripley destroys most of the eggs, enraging the queen, who escapes by tearing free from her ovipositor. Closely pursued by the queen, Ripley and Newt rendezvous with Bishop and Hicks on the dropship and escape moments before the colony is consumed by the nuclear blast.
Back on the Sulaco, Ripley and Bishop's relief at their escape is interrupted when the Alien queen, stowed away on the dropship's landing gear, impales Bishop and tears him in half. Ripley battles the queen using an exosuit cargo-loader, before expelling it into space through an airlock. Ripley, Newt, Hicks and the still-functioning Bishop then enter hypersleep for the return to Earth.
The Sulaco experiences an onboard fire and launches an escape pod containing Ellen Ripley along with Newt, Hicks, and the damaged android Bishop, all in cryonic stasis. During the launch, the ship's medical scans of the cryotubes show an Alien Face Hugger attached to one of the crew members. The pod then crashes on Fiorina 'Fury' 161, a foundry facility and penal colony inhabited by all-male inmates. After some inmates recover the pod and its passengers, an Alien Face Hugger is seen approaching the prison dog.
Ripley is taken in and awakened by Clemens, the prison doctor, and is told she is the only survivor of the crash. Suspicious of what caused the escape pod to jettison and what killed her companions, Ripley requests that Clemens perform an autopsy on Newt. She fears that Newt may be carrying an Alien embryo in her body, though she does not share this information. During the autopsy, no embryo is found in Newt's body. A funeral is performed for Newt and Hicks. Elsewhere, the prison dog enters convulsions, and an Alien bursts from its body.
The Alien soon begins to attack members of the colony, killing several. To get answers, Ripley recovers and reactivates the damaged android Bishop, who confirms that there was an Alien on the Sulaco. She then informs Andrews of her previous encounters with the Aliens and suggests everyone work together to hunt it down and kill it. Skepticial, Andrews explains that the facility has no weapons. Their only hope of protection is the rescue ship being sent for Ripley by the Weyland-Yutani Corporation.
Back in the prison infirmary, Clemens is killed by the Alien, but when it is about to attack Ripley, it suddenly pauses, mysteriously sparing her life. She runs to the mess hall to warn the others, Andrews orders Aaron to take her back to the infirmary, but the alien suddenly ambushes and kills Andrews. Ripley rallies the inmates and proposes they pour flammable waste, into the ventilation system and ignite it to flush out the creature. An explosion is caused by the creature's premature intervention, resulting in several deaths.
Ripley scans herself and discovers the embryo of an Alien Queen growing inside her. She also finds out that the Corporation truly wants the Queen embryo and the adult Alien. Deducing that the mature alien will not kill her because of the embryo she carries, Ripley begs Dillon to kill her; he agrees to do so only if she helps the inmates kill the adult creature first, and he then rallies his men into action. They form a plan to lure it into the foundry's molding facility and drown it in molten lead.
This results in the death all the remaining prisoners, except Morse, who pours the lead. Dillon is the last victim; he fearlessly remains in the mold to distract the Alien long enough, allowing it to tear him apart. The Alien, covered in molten metal, escapes the mold but is quickly killed by Ripley when she turns on fire sprinklers and sprays the beast with water, causing it to cool rapidly and shatter via thermal shock. While Ripley battles the Alien, the Weyland-Yutani team arrives, including a man who looks identical to the Bishop android, claiming to be its creator.
He tries to persuade Ripley to undergo surgery to remove the Queen embryo, which he claims will be destroyed. Ripley refuses and steps back onto a mobile platform, which Morse positions over the furnace. The company men shoot Morse in the leg, and Aaron picks up a large wrench and strikes Bishop II over the head with it. Aaron is shot dead, and Bishop II and his men show their true intentions, begging Ripley to let them have the "magnificent specimen".
Ripley defies them by throwing herself into the gigantic furnace. Sensing danger the alien Queen erupts from Ripley's chest. Ripley holds on to the alien so it cannot escape the inferno. Ripley and the alien Queen fall into the fire to their death. The facility is closed down and the last surviving inmate, Morse, is led away. A sound recording of Ripley (her final lines from the original Alien) is heard from the Sulaco escape pod.