Minor spoilers ahead
The finale of Alien left Sigourney Weaver's Ripley alone and adrift, in a godless universe. She is found, in suspended animation, some 57 years later, alone and abandoned by a Company that holds her responsible for the seemingly inexplicable destruction of the Nostromo and its crew. Ripley is all but labeled a nut case, but her story of a lethal alien life form is taken much more seriously, when the colony set up on the original planet just seems to vanish without a trace. She is talked into joining a group of marines, to find out what happened, on the condition that not only can she get her old job back, but if they run into any of the creatures, they will be wiped out.
Of course, it's never quite that simple; Ripley and the marines discover that all of the colonists -- except for a little girl, named Newt -- have all been cocooned and killed by the aliens. Not long after that, an ambush takes out most of the soldiers and strands them, until they can jury rig and escape plan. On top of all of that, the resident Company guy-in-a-suit (Paul Reiser) is adamant that -- recent history be damned -- they will also smuggle an alien back to Earth, for study.
If you've seen Aliens, then it goes without saying that they more-or-less abandoned the horror sandbox with this one, and went more for hardcore action. If you'll forgive the pun, this has alienated some fans, ever since its release, but you can hardly blame the filmmakers for wanting to go in a slightly different direction. By changing the film's context just a little bit, they were able to keep the concept and aliens fresh and threatening.
Whatever you can say about James Cameron, these days, he was at the top of his game, making this movie (probably the only film he made that was better, was Terminator 2, another sequel that he'd made surpass its predecessor). He stayed true to Ridley Scott's claustrophobic and lived-in universe, without just outright copying the first movie (even if there are bits of the climax that come dangerously close). He also took the time to flesh out Ripley; previous rather cold, she is revealed to be a single mother, whose daughter passed away, alone, while Ripley was floating through deep space. While I have few complaints about the theatrical version of the movie, the sub-plot detailing this fact, is a welcome addition in the extended cut.
The casting of comedian Paul Reiser ("Mad About You") as the human villain, invariably raises some eyebrows, but I've honestly never had any problem with it. Reiser plays the role pretty straight, and he is adept enough of an actor that he believably progresses from a quiet pencil pusher to a full-on slimeball. If you were to show this to someone young enough to not know about his sitcom days, I'd bet you'd be hard pressed to convince them he was ever anything but a dramatic actor.
In general, while there are those who would have complaints about the tonal change, the fact is that Aliens not only set the new standard for the series, but it's undeniable that the series peaked, with this film. The acting is solid, the writing very rounded and gripping, the effects are never better than in this movie, and the creatures smarter, as well as eerie and menacing. If you're any kind of action/sci-fi/horror fan, this is absolutely a movie that you MUST see.