Alien, produced by Gordon Carroll, David Giler and Walter Hill. Alien received both critical acclaim and box office success, receiving an Academy Award for Best Visual Effects,Saturn Awards for Best Science Fiction Film, Best Direction for Ridley Scott. While studying cinema at the University of Southern California, Dan O’Bannon had made a film with director John Carpenter entitled Dark Star. 
Ronald Shusett,Impressed by Dark Star, he contacted O’Bannon and the two agreed to collaborate on their projects, choosing to work on O’Bannon’s film first. O’Bannon wrote twenty-nine pages of a script titled Memory Comprising ” a crew of astronauts awaken to find that their voyage has been interrupted because they are receiving a signal from a mysterious planetoid”.
O’Bannon was working with Alejandro Jodorowsky on his failed film which was later directed by David Lynch (Dune). O Bannon met, Chris Foss, H. R. Giger, Jean “Moebius” Giraud. O Bannon took Foss, Giger and Giraud and brought them into the Alien project. It was the success of Star Wars that allowed Alien to get a green light with Ridley Scott directing and Jerry Goldsmith composing the eerie score.
The second screening of Alien for 20th Century Fox representatives in St. Louis suffered from poor sound quality in the theater. A subsequent screening in a newer theater in Dallas went significantly better.The film was previewed in various American cities in the spring of 1979 and was promoted as “In space no one can hear you scream.” Alien opened in America on May 25, 1979. Audiences lined up for blocks to see it at Grauman’s Egyptian Theatre in Hollywood where a number of models, sets, and props were displayed outside to promote it and sadly, religious protestors set the model of the “space jockey” on fire. Roger Ebert called Alien a piece of trash but later in his soulless life he talked great things about it and even included it as a historical work of art.
“The commercial spacecraft Nostromo is on a return trip to Earth with a seven-member crew in stasis: Captain Dallas, Executive Officer Kane, Navigator Lambert, Science Officer Ash, Warrant Officer Ripley, and Engineers Parker and Brett. Detecting a mysterious transmission, possibly a distress signal, from a nearby planetoid, the ship’s computer, Mother, awakens the crew. Following standard company policy for such situations, the Nostromolands on the planetoid and Dallas, Kane, and Lambert head out to investigate, damaging their ship upon landing in dust. 
They discover the signal is coming from a derelict alien spacecraft. Inside, they find the remains of a large alien creature whose ribcage appears to have exploded from the inside.
On the Nostromo, Ripley determines that the transmission is not a distress signal but a warning. In the alien ship, Kane discovers a chamber containing hundreds of eggs. As he inspects one, a creature springs out, spits acid through his space helmet and attaches itself 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 violates protocol by overriding Ripley’s lock and letting them in. The crew are unable to remove the creature from Kane’s face, as its grip is strong and its blood is an extremely corrosive acid. It eventually lets go, crawls away, and dies.
The crew repair the ship and lift off. Kane awakens and seems healthy, but during the crew’s final meal before re-entering stasis, he chokes and convulses in pain before a small alien creature bursts from his chest, killing him, and escapes into the depths of the ship to molt.
Since attacking the creature with conventional weapons could result in its corrosive blood breaching the ship’s hull, the crew attempts to locate and capture it with motion trackers, nets, electric prods, and flamethrowers.
Brett is sent to look for the crew’s cat, Jones, and the now fully grown alien attacks him and disappears with his body into the air shafts. After a heated discussion, the group devises a plan to jettison the creature out of the ship. Dallas enters the Nostromo’s labyrinthine ventilation shafts, intending to force the alien into an airlock, but it ambushes him. Lambert, realizing the alien is killing the crew one by one, implores the others to escape in the ship’s shuttle. 
Now in command, Ripley explains that the shuttle will not support four people, and recommends that they continue with Dallas’ plan of flushing the alien out.
Accessing Mother, Ripley discovers that Ash has secretly been ordered to return the alien to the crew’s employers, who consider the crew expendable. 
When Ripley confronts Ash, he tries to choke her to death. Parker intervenes and knocks off Ash’s head, revealing him to be an android. Parker reanimates Ash’s head, and Ripley interrogates him. They learn he was assigned to the Nostromo to convince the crew to capture the creature and return it for analysis, even at the expense of the human personnel. Ash taunts them about their chances of survival against the “perfect organism.” Parker turns a flamethrower on Ash.
Ripley, Lambert and Parker agree to set the Nostromo to self-destruct and escape in the shuttle. However, Parker and Lambert are ambushed and killed by the alien while gathering life-support supplies. Ripley initiates the self-destruct sequence and heads for the shuttle with Jones, but the alien blocks her path. 
She retreats and unsuccessfully attempts to abort the self-destruct sequence, then returns to retrieve Jones, finding the alien gone. She narrowly escapes in the shuttle as the Nostromo explodes.
As she prepares to enter stasis, Ripley discovers the Alien is aboard the shuttle. She dons a spacesuit and opens the shuttle’s airlock, causing explosive decompression which forces the Alien into the shuttle’s open doorway. 
She propels it into space by shooting it with a grappling hook, but the gun catches in the closing door, tethering the alien to the shuttle. Ripley activates the engines, blasting the alien into space. After recording the ship’s final log entry, she places herself and Jones into stasis for the voyage home. “https://en.m.wikipedia.org/wiki/Alien_(film)
One of the deleted scenes, which I though was important is when Ripley discovers a “cocooned” Dallas, which Dallas replies with “kill me” leaving Ripley to set fire the Dallas, killing him. 
This was such an important factor to the series explaining that the Xenomorphs keep victims alive to impregnant them and colonize wherever the Xenomophs live. Also there were the deleted scenes that explained the love triangle between Dallas, Ripley and Lambert which was cut in the final product which in my opinion, it would’ve made the storyline more personal.
Alien spawned a legacy of sequels (such as Aliens Ali3n and Alien Resurrection), comics, video games and novels. There is also the prequel Alien:Covenant (directed by Ridley Scott) and the untitled Aliens 2 project directed by Niell Blomkamp starring Sigourney Weaver and Michael Beihn returning as Ripley and Corporal Hicks in the direct sequel to Aliens while avoiding Ali3n and Alien Resurrection all together.
