It’s impossible to discuss Serenity without bringing up Firefly, the Joss Whedon-created cult TV show that preceded Serenity. Firefly was a space western that only lasted 14 episodes, and it was brilliant, but Fox totally bungled its release by airing episodes out of order and showing sports instead of several episodes. Serenity was released in theaters two years after Firefly‘s last episode and served to tie up a few of the loose ends left by the show’s premature cancellation. Serenity was just as enjoyable as the best episodes of the show, mixing that signature Whedon wit with better special effects and higher stakes while preserving the individuality of every character. One of the major appeals of Firefly was that it felt like it could have endless stories to tell, and, magically, Serenity preserved that too.
Quicker take: Like Cowboys & Aliens, but you actually care about it.