"The Alien" refers to the lone Xenomorph that appears in the original Alien film. This creature is endoparasitoid in nature, meaning that it uses other life-forms to reproduce, and is described by the Synthetic Ash to be the perfect killing machine. Non-sentient and unhampered by any human notions of morality, the Alien is driven purely by the instinct to kill, breed and survive.
The Alien began its life as an egg which was discovered aboard a derelict spacecraft discovered on the planet LV-426. G.W. Kane, XO of the USCSS Nostromo, examined the egg up close when it hatched, releasing a xenomorph hatchling that grasped onto Kane's face. Kane's crewmates took him back to the Nostromo where they attempted to remove the facehugger from his head, but the creature's tail tightened its grip around Kane's throat when the crew tried to pull it off. Removing the facehugger surgically proved futile as well as the creature's blood was highly acidic; a single drop of acid blood ate through the deck and several decks below, almost rupturing the hull.
The facehugger eventually let go of Kane of its own accord, dying immediately after. However, what the crew did not yet know was that the facehugger had implanted a parasitic embryo within Kane's body which was rapidly developing. Shortly after regaining consciousness, Kane joined his shipmates for one last meal before they returned to hypersleep. As they ate, Kane suddenly went into violent spasms as the newly-developed alien within Kane began to chew its way out of its host's body, bursting from Kane's chest in an eruption of gore. The small, snake-like chestburster screeched before quickly scrambling off the dining table and darting down a corridor.
The remaining Nostromo crew gathered up whatever weapons and tools they could to try and hunt down the creature, delving into the bowels of the ship. However, they found themselves completely unprepared as the Alien had grown to a full-sized adult within a matter of hours. One by one, the monster killed off each member of the crew, leaving only Warrant Officer Ellen Ripley to fight it off. With no one left to save, Ripley set the Nostromo's engines to self-destruct and headed for the shuttle Narcissus to make her escape from the doomed ship.
Ripley managed to launch the Narcissus and escape before the Nostromo exploded, but unfortunately the Alien had escaped too, boarding the shuttle just as it took off. After donning a spacesuit, Ripley blasted the creature with steam to lure it out, before opening the shuttle's airlock and ejecting the monster into space just moments before it could attack her. However, it clung on in the open doorway and began to haul itself back on board; Ripley shot the Alien through its abdomen with a harpoon gun, flinging it out into space. The harpoon became caught in the closing door, tethering the Alien to the ship, and once again the wounded creature attempted to climb back aboard, this time through the shuttle's engines. Ripley ignited them, incinerating the Alien and finally hurling its remains into the black void of space.