During its return to the earth, commercial spaceship Nostromo intercepts a distress signal from a distant planet. When a three-member team of the crew discovers a chamber containing thousands of eggs on the planet, a creature inside one of the eggs attacks an explorer. The entire crew is unaware of the impending nightmare set to descend upon them when the alien parasite planted inside its unfortunate host is birthed.
The commercial spacecraft Nostromo is on a return trip to Earth with a seven-member crew in stasis: Captain Dallas (Skerritt), Executive Officer Kane (Hurt), Warrant Officer Ripley (Weaver), Navigator Lambert (Cartwright), Science Officer Ash (Holm), two Engineers, Parker (Kotto) and Brett (Stanton) and also a cat named Jones. Detecting a mysterious transmission from the nearby planetoid LV-426, the ship’s computer, Mother, awakens the crew. Company policy requires crews to investigate such transmissions so they land on the planetoid, sustaining damage from its atmosphere and rocky landscape. Parker and Brett repair the ship while Dallas, Kane and Lambert head out to investigate. They discover the signal comes from a derelict alien spacecraft and head inside it, losing communication with Ash. Exploring the spacecraft, they find the remains of a large alien creature whose rib cage appears to have exploded from the inside.
On Nostromo, Ripley determines that the transmission may be a warning, not a distress call. In the alien ship, Kane discovers a chamber containing hundreds of large egg-like objects. As he inspects one, a creature springs out and attaches to his face. Dallas and Lambert carry the unconscious Kane back to the Nostromo. As acting senior officer, Ripley refuses to let them aboard, citing quarantine regulations, but Ash ignores Ripley and lets them in. The crew unsuccessfully attempt to remove the creature from Kane’s face, discovering that its blood is an extremely corrosive acid. It later detaches on its own and dies.
With the ship partly repaired, the crew lifts off. Kane awakens, dazed but otherwise unharmed. During a final crew meal before stasis, he chokes and convulses in pain. He dies as a small alien creature bursts from his chest and escapes out into the ship. Wary of its corrosive blood, the crew attempts to locate and capture it with motion trackers, nets, electric prods and flamethrowers.
While looking for the alien signals with motion trackers, Jones, the crew’s Cat, was accidentally detected and emerges scared from a hole. To prevent further confusion, Brett tries to catch the cat and follows it into an engine room, where the now fully-grown alien (Badejo) attacks him and disappears with his body into an air shaft. After heated discussion, the crew decide the creature must be in the air ducts. They plan to herd it into an airlock to jettison from the ship. Dallas enters and searches the Nostromo’s ventilation ducts but it ambushes him. Lambert implores the others to abandon ship and escape in its small shuttle. Now in command, Ripley explains that the shuttle will not support four people and pushes to continue with Dallas’ plan of flushing the alien out…