Ripley crash lands on a prison planet with a few remaining prisoners who have founded a religious order. The alien has come along for the ride too.
Possibly the bleakest movie to have come out of Hollywood since the 70s. Relentlessy downbeat movie has extraordinary power though. The DVD release includes a Special Edition, an assemblage of the material as it was intended (although still disowned by Fincher). This version is much, much better, still just as bleak, but at least the narrative works better courtesy of a subplot involving Golic and the alien. So my advice watch the special edition but flip versions for the smelting plant finale as the final shots are more ambitious and carry more resonance.
In hindsight, and I hated this movie the first time around, it is a fitting end to the trilogy, emphasis on the word "end". The subsequent movie, while entertaining enough, is fluffy-bunny-lite in comparison and has plot twists that defy logic. And the less said about the Alien vs Predator match-ups, the better.
See also Alien (1979), Aliens (1986), and Alien: Resurrection (1997).