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En route to Earth, the computer of the ship Nostromo awakened the crew from hypersleep: the ship has detected an apparent distress signal from a nearby planet. Under the terms of their contract, the crew must investigate. Upon landing on the planet, they send several of their company to find the ship and any possible survivors. They eventually reach an apparently ancient spacecraft. Upon entering, the crew find the corpse of the ship's non-human pilot with a large gash along its stomach. After more exploring, they find a massive chamber full of eggs. They send one of the crew (Kane) down to investigate. Upon closer inspection, one of the eggs hatches slowly, and an unknown lifeform attaches itself to Kane's face.

Story of the movie (spoiler)

The eponymous alien creature, a lethal predator with consistently exotic abilities and physical attributes, reproduces by parasitizing living victims with embryos; or by transmuting them directly into the early alien reproductive stages (egg and facehugger) through unknown means. The film crew set up the plot device of the alien having acid for blood in order to prevent the Nostromo's crew from readily killing it with firearms — the spilled blood would have eaten through the ship's hull. On the other hand, the crew think a flamethrower a suitable weapon, as Ash explains: "Most animals retreat from fire". Commentators have compared the life cycle of the alien to that of the ichneumon wasp — due to its parasitoid nature.

After the alien captures the ship's captain, Dallas, during a botched attempt to trap the creature, Ripley assumes command. She discovers that the Company that owns the ship had deliberately re-routed it to investigate a non-human distress signal and to return a specimen. We learn that the Company placed the Science Officer, Ash (revealed as an android), on short notice at the departure colony Thedus with the purpose of protecting the creature, and with instructions to regard the crew as "expendable". Ripley — as the sole survivor of the Nostromo — initiates the ship's self-destruct sequence, escapes in a shuttle craft with a cage carrying Jones, the ship's cat, and finally destroys the alien by blowing it out of the airlock into open space.