This came out in 1979, when I was like 3 and a half years old. I'm writing this review after watching it on DVD in 2017. Before watching the DVD, I felt fairly sure I had seen it on TV, or something, like sometime in the 80s. After watching the DVD, I'd say the odds are roughly even as to whether I'd seen it before or not. I definitely saw the sequel at some point, because there are some memories I have that are specific to that movie. But I feel like anything familiar to me about this movie could just as easily be memories I have from the sequel. So... I really don't know. (Edit: I was waiting to watch the second movie to say something for sure, and now I think it's safe to say that I probably did see this movie, just because if there's one thing in my memory more likely to have come from this movie than the sequel, it's the end where Ripley is just in her skivvies as she prepares to go back into stasis.)
Anyway, this movie, and the franchise it spawned, is surely one of (if not the) most iconic example of sci-fi horror movies. (I mean, assuming you exclude "Frankenstein," which is technically science fiction... but not really "sci-fi," to my way of thinking.) Oh... and I should mention the famous tagline, "In space, no one can hear you scream." If you've heard that before (and how could you not?), this is where it's from. So... I wish I could say I loved the movie, instead of just "rather liking" it. But for me, this is the kind of movie I appreciate more for starting a great franchise than I do in and of itself. I like the main character, Ellen Ripley (Sigourney Weaver), though I probably liked her more in the sequel. I liked the design of the alien, but probably liked it more in the sequel (which I think is probably scarier than this movie, because there were more aliens). And I like the fact that this movie pretty much ushered in the idea that true sci-fi/horror hybrid movies could be vastly superior to the mostly schlocky B-movies of old. But on the other hand... I'm ashamed to say that when it comes to movies (more so than novels), I tend to be somewhat easily underwhelmed by science fiction that isn't more spectacular. (An expectation ushered in, as far as I'm concerned, two years before this movie by the start of an even greater franchise.)
Anyway, it's about the 7-person crew of the commercial starship Nostromo, which is currently returning to Earth, transporting cargo from a deep space mining mission, I guess. The crew is in stasis, but the ship's computer, "Mother," awakens them early because it's intercepted a signal of unknown origin, which protocol requires them to investigate. The captain's name is Dallas (Tom Skerritt, whom I mainly know from Picket Fences). I guess the first officer was Kane (John Hurt). Ripley was, I think, the third in command. The rest of the crew includes the navigator, Lambert (Veronica Cartwright); the science officer, Ash (Ian Holm); and engineers Parker (Yaphet Kotto) and Brett (Harry Dean Stanton). So... the Nostromo heads for the source of the signal, then a shuttle detaches from the ship to land on the planet's surface. Dallas, Kane, and Lambert put on spacesuits and leave the shuttle to investigate the source of the signal, which is some kind of alien ship, I guess. They find a long-dead pilot. And Kane finds a whole bunch of creepy-looking eggs. One of them opens up, and a a creature known in fandom as a "facehugger" jumps out and attaches to his face. Dallas and Lambert bring him back to the shuttle, but Ripley doesn't want to let them in, because protocol forbids it (since it could mean allowing the rest of the crew to become infected). However, Ash ignores this protocol, and lets them in, ostensibly out of concern for Kane's life. And at some point the shuttle returns to the Nostromo, but I forget exactly when.
Anyway, when they bring the comatose Kane to the infirmary and try to detach the facehugger, they find that its blood (or whatever) is a highly corrosive acid that eats through several levels of the hull. But eventually, before they can figure out what to do about Kane, the creature disappears (conveniently enough, when no one is in the infirmary, and for some reason there's no cameras trained on the patient). They search for the alien, and eventually discover its dead carcass. Subsequently, Kane wakes up and seems okay. They all decide to have a meal before going back into stasis and returning to Earth, but in the middle of the meal, Kane suddenly goes all like epileptic, or something, and then a creature bursts out of his stomach, leaving Kane dead. (The whole bursting from the stomach thing is an iconic scene, but one I'm sure could be seen in other movies in the franchise.) After the small creature skitters away, the remaining crew search for it, hoping to capture it (without potentially puncturing it and letting any of its acid blood out). At one point, the crew's cat, Jones, sets off the motion-sensing device they're using to look for the creature, and then runs away. So Brett has to go look for Jones, to make sure he doesn't accidentally set off the sensor again. (It strikes me as a very bad idea for him to go alone.) He finds a shed skin left behind by the creature, which we soon see has grown way more than could reasonably be believed after just one stage of molting. (I gotta say, the species' apparent reproductive process seems weirdly complicated to me. First lay eggs, from which hatch one form, that apparently implants a smaller form inside a host, which soon violently emerges from host, then morphs into its final form? Why so many damn steps? It's almost like nature designed it to star in horror movies.) Oh, as for the signal, I don't think it was ever fully decoded, but it was suggested at one point that it was a warning. But what or why, I don't know, because it looked to me like the dead pilot from the derelict ship was the same species as the alien now terrorizing the Nostromo crew, but maybe I'm wrong about that. I can't imagine the same species needing to warn anyone to stay away. (And it's clear that the pilot had a creature burst out of its own gut the way this one did from Kane's, so... that probably wouldn't happen with the same species, either, I'm guessing.)
Well, lots of other stuff happens, but I don't want to spoil any specific details. I'll just say that the movie, while being revolutionary in some ways, it also has plenty of horror tropes, like "cat scares" and of course a "final girl." (Oh, and there's a revelation about one of the characters that I won't spoil now, but which will definitely play into the sequel.) And... I guess that's all I want to say. It's a decent movie, fairly scary, and, you know... a classic of both the science fiction and horror genres.