The story is similar to the original. There's a space colonization ship carrying 2000 humans (in cryostatis) with 15 crew members. The ship gets hit by a random solar flare in space and it messes up the ship. Walter (our good AI, played by Fassbender) wakes them up and has them fix the ship. They realize that they have to go back to cryostasis for 7 years before they reach their colonization planet destination. They hear a human voice through a transmission and they pinpoint where its coming from. Turned out there was a planet capable of sustaining life that they didn't know about (somehow, mysteriously) so the captain decides to go there. You know what happens next.
They land on the planet, walk around and touch a bunch of plants and go in the caves where Elizbeth Shaw was last seen in (the main character from Prometheus). This was the planet where David (our evil ai, also played by Fassbender) was last at in Promethus. So some crew members get infected by the xenomorph toxin (virus like subsatance) and they get sick. Then the xenomorphs rip them apart and all hell breaks loose.
They're fighting a xenomorph at night when all of a sudden a guy in a hood (with long locks) shoots a flare out into the sky. The xenomorph gets scared and runs off. It turns out to be David. What happens next is the best part of the entire movie. David meets Walter. We find out that David has been genetically engineering xenomorphs and that when he came to the planet he killed all the primitive humans that lived there with the xenomorph virus. Back then it was in an almost infant-like state so it didn't create xenomorphs, we just see a scene of it just killing the inhabitants of this human-like planet.
There's some homoerotic stuff between David and Walter. David kisses Walter at some point. He shows him how to play the recorder and he says,"I'll do the fingering". Kind of funny that they gave an android some sexual elements. There's also a scene where David is trying to kill the female lead role and she asks,"what are you going to do with me"? And he responds,"the same thing I did with her". And then he pushes her on the table, gets on top of her, and then starts to kiss her. What was that implying?
There's a fight between David and Walter. We see Walter about to smash David's head in with a rock but then it ends scene. We don't see Walter kill him so its up to the viewer to make of it what they will. Walter ends up back on the ship with the remaining living crew and they find out an alien infected a guy who got attacked by a face attaching xenomorph. He didn't die but his face was badly burned. Somehow this strain of xenomorph still infects you even if it doesn't kill you. So they have to kill this xenomorph and it turns out to be the standard alien we all know and love. Tall, black, sharp pointy tail, second mouth to feed with. All that jazz.
Our last two living crew members end up shutting it down by releasing it into space. They actually shoot out a giant truck with a spear-like front end and it ends up cutting the xenomorph right through its body, ending the madness.
Then Walter puts the pilot (Tennessee, played by a guy who's shown up in more comedies than science fiction films) into cryostasis. Its the lead actress's turn to go into cryostatis. She's right about to be put to sleep but then at the last second she realizes that something is wrong. What did she realize? Still not too sure about that one. She starts panicking super hard and starts pounding on the class of her chamber. But its too late. Walter puts her to sleep, and then goes to the human embroyo compartment. He takes out four xenomorph embroyos from his mouth and puts them in there. Good luck with your colonization humans.
The main question is the obvious one. When David was sent out by Weyland corp did he have an objective or did he simply become a crazed android that wanted to evolve the universe and create perfect beings because humans were not that? Another thing to point out that's never been explored before in an Alien movie is that when a xenomorph attacked a crewmember David talked to it and said,"calm down, breathe". The xenomorph actually stopped and breathed, talking in its language. Then David said something like,"once you get them to trust you they'll do anything you want for the rest of their life". Boy, so these things can be trained like pets? Mommy, I want one too!
Quite a wild ride of a movie. Everybody pretty much dies and its implied that all 2000 people (in cryostatis) aboard the ship are probably going to get killed by those xenomorph embroyos. Who knows, maybe in a another Alien movie we'll see Walter or some other android controlling them like his pets. Like a summoner character in a video game. That would be fun. The blood frenzy was real in this movie too. You should definitely check it out if you haven't seen it!