The crew of the mining transport ship Nostromo are awoken from hypersleep when the ship's computer makes an unscheduled diversion to a planet 39 light years from Earth to investigate a distress beacon. After a near-disastrous landing, the crew investigate the wreck of an alien spacecraft underneath which is a huge cavern filled with large eggs. When one of the crew examines an egg, a horrifying creature leaps out and attaches itself to the man's face. Returning to the ship, the thing makes a bloody escape and now the survivors must contend with a creature of unknown powers that is stalking them one by one through the ship's corridors.
Alien is the other side of the science fiction boom set off by Star Wars in 1977. Where director George Lucas gave audiences a science fiction universe filled with spectacle and adolescent adventure, Alien's Ridley Scott provided a Lovecraftian universe of random, meaningless horrors. It also introduced a continuing stereotype into cinema: The space-faring working stiffs who forsake jump suits for overalls and have the emotional maturity of an excessively retrograde twelve-year old. It's become such a cliché that after thirty years one looks wistfully at the clean-cut professional spacemen that Doctor Who used to run into back in the '70s. But in 1979, this was a real innovation. Instead of a group of explorers or soldiers, our heroes are a collection of ship hands called upon to deal with a situation they are completely unprepared for. This gave the story a more powerful sense of fear and unpredictability as the characters made the sort of mistakes a better trained group would have avoided as a matter of course.
It was also the first film to benefit from Star Wars's establishing that audiences didn't need to be sold a sci fi concept and that form only needed to follow pseudo-function. No sane engineer or designer would build a craft like the Nostromo. The sets of Alien are incredibly rich with all manner of knobs, buttons, lights, computer screens, and all the accoutrements of the future, but the construction is amazingly clumsy with cables snaking around for no reason, only a vague attempt at some sort of logical purpose for this or that device, and a weird mixture of periods with high-tech computer panels sitting side by side with radio boxes like something out of a Lancaster bomber. Where previous set designers, such as those working on 2001: A Space Odyssey, tried to come up for a rational purpose for this hatch or that console, Alien tosses it all out the window in favour of supporting an overall, distinctive look. Take a glance at any sci fi film since then and you can see how far this trend has gone.
The titular alien himself is equally without logic. From a biological point of view, it makes no sense at all and even all the back-filling of the impossible to kill off sequels haven't been able to stop up the gaps. However, Scott does an excellent job of keeping the story going so that the audience doesn't notice this–at least, not right away. Nor do they notice that the plot is filled with holes of its own or that it looks a bit too much like It! Terror From Beyond Space, Planet of the Vampires, and Van Vogt's "Discord in Scarlet" to be a coincidence.
But you shouldn't be too hard on the writers. Dan O'Bannon came up with the idea for Alien partly because he hated how the alien in his previous film Dark Star looked patently like a beach ball. What a pity he didn't realise that he was the first man who managed to parody himself before the object of the parody had even been imagined.